With Xen it is possible to setup multiple virtual computers on one physical computer running Linux. This has a number of advantages:

• Each guest can be running a different OS. For example some can be running Debian, others can be running Centos.
• Each guest can be administrated seperately.
• Each guest can have different network access.

There are some reasons that are not good reasons for choosing a virtual host over a real host.

• Security. There may be security holes that allow you to access the root domain (DOM0) from within a guest domain. Once an attacker has root access to the root domain, he/she has access to everything.
• Less administration. Each virtual box still needs to be administrated seperately, so using Xen doesn't remove any of the administration tasks required.

I have documented some of my Xen experiences.