So, OpenVZ vs. XEN
From a client perspective, we can't really see the substantial benefits of using OpenVZ over XEN. Hopefully the information provided below will give this statement some weight, but this is ultimately dependant on what the client uses his virtual server for, and their budget.
Typically, XEN is a more expensive choice of virtualization platform, this is mainly, if not solely, because the host cannot oversell their hardware. A host that uses OpenVZ for it's backend always has the opportunity of allocating more resources than can actually be used.
Example OpenVZ: A host has a quad-core node with 8GB of memory and a 500GB harddrive. This can be sold as a quad-core node, with X amount of memory and X amount of harddrive (X representing any figure imaginable). This basically allows the host to assign more resources than what the customer can use, and results in a throttled service for all users on the node.
Example XEN: A host has a quad-core node with 8GB of memory and a 500GB harddrive. This can only be sold as the above and that's it. Once the resources have been allocated, XEN doesn't allow the host to create any more virtual servers.
Other differences include:
OpenVZ uses a 'burstable' approach to it's memory management. When a host creates a virtual server and they come to the memory management area of the setup, they have the opportunity of allowing that particular VPS to use more physical memory when needed. This is extremely useful, however this is totally dependant on whether the host node has the memory available for the virtual server to use.
Xen uses a SWAP space approach when a VPS has run out of memory. This basically means that idle processes are pushed to allocated harddisc storage. This in turn lessens the struggle put on the memory load, and frees up usable space to be used on physical RAM.
XEN is a completely isolated environment, this means that it acts virtually identical to a dedicated server. All CPU resources and memory limits are allocated to each virtual server.
OpenVZ operates a 'friendly' approach to sharing it's resources across all of the virtual servers on each node. This means that if you have another user on the node using more resources than they should, it will effect the performance of your virtual server.