Xen is virtualization software that enables the parallel operation of multiple virtual machines on a physical computer. The software originates from a research project conducted at the University of Cambridge.
Xen can be used for paravirtualization as well as in the area of true virtualization.
Modification of the guest systems is necessary on systems that do not offer hardware-based virtualization to allow them to run under Xen. These modifications enable the guest operating systems to cooperate with Xen, in which case paravirtualized systems operate with virtually no loss of speed. The relevant patches are available for a range of popular operating systems, such as Linux, OpenSolaris, and the BSD derivatives.
Xen also enables true virtualization with the presence of hardware-based virtualization functions, as can be found in modern processors. This has the benefit that the guest systems do not need to be modified.
Xen Hypervisor operates at the lowest level. It divides the virtual machines, which are referred to as domains. It is also responsible for managing processes, resources, and interrupts. The host system, Domain-0, is responsible for managing the guest systems as well as the use of the hardware. The outstanding hardware support of the Linux kernel, which is compatible with Domain-0, therefore also benefits all guest systems. The guest operating systems run exclusively as user domains (DomUs).
Alternatively, there is also the option of operating NetBSD or OpenSolaris in Domain-0.
Xen is free software; the source code is freely available on the project page .