I downloaded the Xen 2.0 Demo CD Beta 2 and tested it. Looks pretty cool, however for the GUI it needs to run on a system with 512MB RAM, only my laptop has that much. I also tried it on a Pentium II Dual CPU Dell PowerEdge server and it wouldn't run. I think the CD-Rom in the PowerEdge may have not been up to snuff.
I am trying to copy out the Xen kernel and boot files + root file systems over to my laptop's Debian install. I think it would save me some time.
I have in the past worked some with coLinux and User-Mode-Linux however never got to where I would have liked. coLinux worked great under Windows, thats a simple download and install, however I had issues each time I tried to build a custom Linux kernel which would be the host kernel for coLinux.
I would like to be able to have virtual machines under Linux for testing different items where I can easily backup machine before adding something new, and keep things seperate from each other. Nagios 2.0 is one of the applications I would like to run in a virtual Debian system.