Definition: Xen

Open source virtualization software that is used to partition workstations and servers into separate virtual machines, each containing its own copy of an OS. Pronounced "zen," and developed at the University of Cambridge in the U.K., Xen is noted for its fast response and low overhead. Xen is a small, low-level "hypervisor," which is the first control software loaded when the computer starts up.

Originally developed for x86 machines, support for the IA-64 (Itanium) and POWER5 (Mac) platforms followed. Xen began gaining wide acceptance in the Linux server market in the 2005 time frame.

The OS Must Be Ported
Xen uses one or more privileged "guest" operating systems for handling the actual device drivers for the hardware. Known as "paravirtualization" and unlike VM environments where the OS runs as is, the OS that runs on top of Xen must be programmed to call Xen virtual drivers, which, in turn, call the real drivers. Linux and versions of Unix were the first operating systems ported to Xen.

Because the real drivers run outside of Xen, the machine can always be booted into a consistent, secure base personality. In addition, a virtual machine (OS and apps) can be moved to another server and continue running in a matter of seconds.

No Porting for Hardware Virtualization
There is no requirement to port the operating system to Xen if the hardware platform offers support for virtualization, such as Intel's VT, AMD's AMD-V and IBM's POWER architecture. See XenServer, XenSource, virtual machine, VT, AMD-V and paravirtualization.

A Xen Virtual Machine

Xen uses one or more privileged guest operating systems for driver control, and other operating systems communicate via Xen virtual drivers.