Openvz vs xen
OpenVZ:
Advantages: allows overselling. Very light weight. Can accommodate more Virtual Machines in a server.
Disadvantage: There is no per vps swap.
Why this is important:
OpenVZ will KILL your application if it goes beyond the limit, and this can cause some trouble. There are people out there who want to host oracle on a 64MB vps, and with such customers, using openVZ will lead to constant application crashes, which ultimately will be blamed on the provider. (This is actually something that is common with openvz/virtuozzo hosting in general; you can check some threads at wht).
With Xen, each vps has its own swap, and thus you get an EXACT dedicated server like environment, but with lesser resources. So here, the customers applications will NOT crash, but rather it will become slower. Also, majority of the applications, like apache, spamassassin expects a lot of memory, and openVZ makes memory a very valuable commodity.
So generally my recommendation is that: For friendly customers use openVZ, and use a lot of burst memory. For not-so-friendly customers, use Xen. And that is why we are providing transparent migration. You can start a customer on openVZ, and see how it works out, and if he is getting too many application crashes, you can move him to the SAME configuration on Xen, and he should be able to do fine, though his application would be slower.