Why two hypervisors from Sun : xVM Server & xVM Virtualbox ??

This is the question arising in mind for me & many : "Why has Sun two hypervisors? When should we use xVM server? When should we use xVM Virtualbox?"

I find this question a little bit strange. No one asks: When should we use VMware Workstation, when should we use VMware ESX, and what´s with VMware GSX ... sorry ... VMware Server.

So the difference is simple: Use xVM Virtualbox for your virtualisation needs on your desktop and use xVM Server for the virtualisation in the datacenter. It´s as simple as that. Future developments in both classes of virtualisation may give us a area where both systems are viable ways to do the job (e.g. there are plans to integrate live migration to VirtualBox), but at the moment this should give you a guideline. Of course you could use xVM Server on your desktop system and VirtualBox on your server, but that aren´t their primary use cases.