VirtualBox vs. Bare to the medal VMs?

Jono writes...

Hey Guys,

I use VirtualBox for desktop testing and I have a VMWare ESXi machine
for hosting a few different servers.

I want to replace the VMWare machine with something open source, so I
have been messing with Xen and KVM a bit. I am wondering if you could
talk about these and other such alternatives - things that run close
to bare metal. What is your experience with them? And best ways to
administer them. There are tons of GUIs and command line wrappers.

Or maybe you think VirtualBox is good enough that performance is
comparable to a type 1 hypervisor. I know you've talked up headless
mode before.

Also it would be interesting to talk about isolating resources to
virtual machines for exclusive use. Like reserving one of the
processors for a single specific VM perhaps.

Thanks!