I think Xen is the best technology for virtualization you can use because it is very efficient. I have a server dual xeon with 3 gb of ram running 8 virtual machines with very good performance. The installer of Elastix support xen paravirtualizaion you might want to read this thread:
Xen is really cool and fast, but it is not that easy to learn. If you want something easy, but not that efficient, you can try Virtual Box.