Xen is a technology that permits one physical computer to act as two or more virtual machines (or domains). Each domain is isolated from other domains, so administration privilege can be safely delegated; you can designate a system administrator for one domain and give him the root password for total control of that system, confident that he will not be able to touch the configuration of other domains.

Virtualization technology is also very helpful when testing multiple software versions or configurations, and since virtual machines can be migrated between physical systems, it provides a lot of flexibility for server deployment and management.