This article is a great mini-howto on getting Xen deployed quickly on Fedora Core 3. Testing changes in server configuration is one useful application of virtual machines like those provided by Xen. Unlike VMWare, the operating systems on the host and guest machines must support or be modified to support the Xen architecture. In other words, existing operating systems without support for the Xen architecture cannot be used on guest machines under Xen. This seems to be the result of a decision to sacrifice true virtualization for increased performance, and the results from performance comparisons of applications running on guest machines and those running in native environments are very exciting.