OpenVZ and KVM are both made in such a way that you can over-commit CPU & Memory, however KVM is made so that you *can not* over-commit storage. So it is pure myth that KVM boxes are less oversold than an OpenVZ box. The benefits of KVM would be the fact that you can utilize your own custom Kernel (as with Xen), though for more this isn't exactly necessary. Also, people speak as if overselling is the devil... No, there wouldn't be a VPS industry without overselling. I recently did a study with a test node with OpenVZ (gave out a bunch of free VPS).
I was able to over commit a VPS node that had an E3-1231v3 CPU and 32GB of Memory. The node has committed 150GB of memory to 300 virtual machines, however at peak hours the system only utilized about 15-16GB of system memory. This shows that many people that get VPS don't fully utilize them or they buy them to sit there and rot. Now, with numerous benchmarks I found absolutely no performance degradation either...
OpenVZ is a great platform if you don't have the specific need to manage your own Kernel, and is has much less overhead than that of KVM of Xen. In fact, you will notice that the majority of providers no longer even utilize Xen due to its excessive overhead. For best performance, just pick and choose your hosting provider rather than the Virtualization platform that they utilize, unless your specific needs are that you must have a custom kernel.
