The best free VM hypervisor?

If you’re like me, you’ve probably just adopted some VM because it was integrated with whatever jumpstart/kickstart/other automatic install system that you’ve been using. Well, recently I branched a little outside my element and needed to start researching VMs for running Windows on top of (yeah, the opposite of most of the world, I’ll grant you, but it is what it is).
So, what, thought I, is the best VM for running windows 2003 or 2008 on for some services that we are pondering running for Windows clients? Maybe they are all about the same, maybe not. I didn’t know. I found a really great URL out there that compares the common ones on CPU, memory, IO, and even a database benchmark, very thoroughly!

Summary: if you want to to run a medium-heavy DB server on a VM, don’t do it on Xen, but VirtualBox is good. If you want to do heavy duty CPU stuff with little overhead, Xen might be a fair choice. If you want to do a lot of memory, depending whether it’s throughput or latency oriented, you should probably compare carefully.

http://www.ilsistemista.net/index.php/virtualization/1-virtual-machines-performance-comparison.html?start=18