A group of researchers at Clarkson University produced an absolutely interesting paper about a rarely analyzed aspect in virtualization technologies comparison: virtual machines isolation:

...how well do different virtualization systems protect VMs from misbehavior or resource hogging on other VMs? In this paper, we present the results of running a variety of different misbehaving applications under three different virtualization environments VMware, Xen, and Solaris containers. These are each examples of a larger class of virtualization techniques namely full virtualization, paravirtualization and generic operating systems with additional isolation layers. To test the isolation properties of these systems, we run six different stress tests - a fork bomb, a test that consumes a large amount of memory, a CPU intensive test, a test that runs 10 threads of IOzone and two tests that send and receive a large amount of network I/O. Overall, we find that VMware protects the well-behaved virtual machines under all stress tests, but sometimes shows a greater performance degradation for the misbehaving VM. Xen protects the well-behaved virtual machines for all stress tests except the disk I/O intensive one. For Solaris containers, the well-behaved VMs suffer the same fate as the misbehaving one for all tests.

Read it here. It's a real real real pity this wonderful comparison didn't included Microsoft Virtual Server. This could be a critical point in products comparison, apart any marketing talk. I hope to see a second version soon.