Using a Xen LiveCD is a great way to start using Xen: if you have read about Xen, but are unsure if it is for you, you can use a LiveCD to roadtest the features of Xen, without writing to persistent storage. This lets you decide if Xen is the solution for you, before doing any harddisk partitioning, or formatting. Just remember not to spend too long configuring, as changes are lost when the server is restarted!
The Xen LiveCD is now unmaintained, and slightly out-of-date. Some features of the current Xen release are not included in the LiveCD, however it can still be a useful starting point.