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Xen facts for kids

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This page is about the virtualization software. For other uses, see Xen (disambiguation).
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Xen
Xen hypervisor logo black.svg
Xen screenshot.png
Xen running NetBSD and three Linux distributions
Original author(s) Keir Fraser, Steven Hand, Ian Pratt, University of Cambridge Computer Laboratory
Developer(s) Linux Foundation
Intel
Initial release October 2, 2003; 21 years ago (2003-10-02)
Stable release
4.18.0 Edit this on Wikidata / Error: first parameter is missing. ()
Written in C
Type Hypervisor
License GPLv2

Xen (pronounced "zen") is a special kind of software that lets you run many computer operating systems on the same computer at the same time. Think of it like having one super computer that can pretend to be many smaller computers, each running a different operating system like Windows or Linux.

Xen is free and open-source, which means its code is available for anyone to see and use. It was first created by researchers at the University of Cambridge. Now, the Linux Foundation helps develop it, with support from big companies like Intel and Amazon.

How Xen Works

Xen is a type-1 hypervisor. This means it runs directly on the computer's hardware, even before any operating system starts. It's like the boss of the computer's resources.

Managing Virtual Computers

Xen's main job is to manage all the virtual computers, also called "domains."

  • It handles their memory and decides when each virtual computer gets to use the main computer's processor.
  • Xen starts a special virtual computer called "dom0." This dom0 is the only virtual computer that can directly talk to the computer's hardware.
  • From dom0, you can control Xen and start other virtual computers, called "domU." These domU virtual computers don't have direct access to the hardware.

Different Ways to Run Operating Systems

Virtual computers (domU) can run in a few ways:

  • Traditional Operating Systems: Some operating systems, like Microsoft Windows, can run without changes if the computer's processor has special features for virtualization (like Intel VT-x or AMD-V).
  • Paravirtualized Operating Systems: Other operating systems are changed slightly to know they are running inside a virtual machine. This helps them work more efficiently with Xen.

Xen usually starts from a bootloader (like GNU GRUB). Then, it loads a paravirtualized operating system into the dom0, which then helps manage everything else.

Xen's History

Xen started as a research project at the University of Cambridge in the UK. Ian Pratt and his student Keir Fraser led the project.

Key Milestones

  • 2003: The first public version of Xen was released.
  • 2007: A company called Citrix Systems bought XenSource Inc., which was a company created to develop Xen further. The Xen Project then moved to a new home online.
  • 2007: Xen was shown working on ARM processors, which are often found in smartphones and other small devices.
  • 2013: The Xen Project joined the Linux Foundation. This helped it grow as an open-source project. A new name, "Xen Project," was created to show it was different from commercial products using the "Xen" name.

Since Linux kernel version 3.0, Xen is built right into the main Linux kernel, making it easier to use.

What Xen is Used For

Xen is very useful for many things, especially in the world of cloud computing and servers.

Common Uses

  • Cloud Services: Many companies that provide internet hosting services use Xen. They use it to offer virtual private servers (VPS) to their customers. Companies like Amazon EC2 and Rackspace Cloud have used Xen to power their services.
  • Server Consolidation: Businesses use Xen to run many virtual servers on one powerful physical server. This saves space, energy, and money.
  • Quick Setup: It allows new virtual servers to be set up very quickly when needed.
  • Staying Online: If one part of a system fails, Xen can help move a virtual machine to different hardware, keeping things running without stopping. This is called "live migration."
  • Security: Virtual machines can be kept separate from each other, which helps with security. If one virtual machine gets a problem, it's less likely to affect others.
  • Development and Testing: Programmers use Xen to test new software or operating systems. If something goes wrong, it only affects the virtual machine, not the main computer. This is like having a safe "sandbox" to play in.

Types of Virtualization

Xen offers different ways to run guest operating systems, depending on how they interact with the hardware and Xen itself.

  • PV (Paravirtualization): The guest operating system is changed to work directly with Xen. This is very efficient.
  • HVM (Hardware Virtual Machine): The guest operating system runs without changes, using special features in the computer's hardware to help with virtualization.
  • HVM with PV drivers: This combines HVM with special "paravirtualized drivers" for things like input/output (like hard drives and network cards). This makes HVM guests run much faster.
  • PVHVM: This is a mix, using both paravirtualization and hardware help.
  • PVH: This is a newer way to run paravirtualized guests that also uses hardware help when available.

Xen was one of the first to use paravirtualization, which means the guest operating system knows it's virtualized and works with Xen to be more efficient. This was especially helpful for older processors that didn't have special virtualization features.

For newer processors with hardware virtualization features (like Intel VT-x and AMD-V), Xen can run unmodified operating systems like Microsoft Windows. This is called HVM. Xen uses a program called QEMU to pretend to be hardware for these virtual machines, especially during startup.

Features of Xen

Xen has some cool features that make it powerful:

  • Live Migration: Administrators can move a running virtual machine from one physical computer to another without anyone noticing it stopped. This is great for balancing workload or doing maintenance without downtime.
  • Scalability: Xen can handle a huge number of processors and a lot of memory. It can work with up to 4095 physical processors and many terabytes of RAM.

Where Xen Can Run

Xen can run on many different types of computer processors:

  • Intel: IA-32, x86-64 (common desktop and server processors)
  • ARM: Found in many smartphones and smaller devices.
  • MIPS: Another type of processor, though this support is experimental.

Operating Systems That Use Xen

Xen can be part of a special virtualization platform or included with standard operating systems.

Hosts (Operating Systems Running Xen)

Many popular operating systems can run Xen as the main "host" system:

Guests (Operating Systems Running Inside Xen)

Most operating systems that run on PCs can run as a Xen HVM guest. Some can also run as paravirtualized guests:

Even Microsoft Windows can run as a guest in Xen, especially with hardware virtualization. Companies like Citrix have also made special drivers to help Windows run faster inside Xen.

Managing Xen

There are many tools available to help people manage Xen and its virtual machines. These tools make it easier to set up, start, watch, and stop virtual computers.

  • OpenNebula
  • OpenStack
  • Apache CloudStack
  • Xen Orchestra

Images for kids

  • Xen hypervisor logo black

    The Xen Project logo.

  • Xen screenshot

    A screenshot showing Xen running different operating systems.

See also

  • CloudStack
  • Kernel-based Virtual Machine (KVM)
  • OpenStack
  • Qubes OS
  • Virtual disk image
  • VMware ESXi
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