Quoting Craig Sanders (cas@taz.net.au):
for server-type VMs, though, i'm inclined to use KVM, which is great for running dozens or hundreds of VMs on one server.
I concur, and, shortly after my wife and I get back from being on holiday, I'm going to finish constructing a KVM-based replacement for my antique server (the PIII-based machine at my house in Menlo Park, California that runs linuxmafia.com aka unixmercenary.net, etc.). In testing, KVM has proven to have stunningly low overhead for server use. My main effort before completing migration will actually be on the host-OS side: I need to test and see how much hardening, starting with as much as feasible of grsecurity and PaX, is compatible with supporting KVM. (When I say 'on holiday', I mean from *cough* Australia -- but sadly not Victoria, this time: My trans-Pacific cruise ship arrived in Sydney this morning. My wife and I will be visiting NSW until the 20th. Still, in solidarity with my Melb. friends, I hoisted a VB after completing the Sydney Harbour Bridge climb this morning. Excellent as always.)