Nowadays there are two most popular virtualization technologies – Xen and OpenVZ. We have already talked about both and compared them, so to make the long story short, we are just going to tell, that Xen technology supports full virtualization and a wide range of operating systems. It does not have any inter-carrier shared resources, what makes the work of each server autonomous (there is even a separate /swap partition created under each virtual machine).
OpenVZ is a specialized Linux-solution, which is faster but it lets the carrier share resources (RAM/CPU), what may cause overload issues to both virtual and physical machines. On the other hand, resource sharing allows OpenVZ users to get temporary RAM-boost – so-called burst-RAM.