server virtualization: hypervisor (xen) or virtualbox?
Solution 1:
The best solution for me is an Hypervisor for server virtualization, and the best Hypervisor in Linux enviroment is KVM ...
http://www.linux-kvm.org/page/Main_Page
Solution 2:
Of courz hyperviser virtualization is best for Server side. You have to go for VirtualBox only if you have are using it for Workstation purposes.
You can try to convert your Physical Linux installation to Xen using some tricks http://www.olivetalks.com/2008/02/16/xen-p2v-conversion-in-seven-simple-steps/
The guide is for Cent OS conversion. For debian some commands will be different (or configuration files)
Solution 3:
You should first determine if your processor supports some of the advanced virtualization features, which greatly affects performance. See KVM's FAQ, and this VirtualBox topic. I'm not sure there is much point in running KVM if it is not supported by the CPU.
VirtualBox is easier to use. You can be up and running your Windows guest in 20 minutes, whereas on Xen/KVM you'll probably need to learn about concepts like configuring a network bridge, and fun with /etc/networking/interfaces. I believe VirtualBox requires a GUI, whereas KVM & Xen do not.
Hypervisors like Xen & KVM will provide better performance, but performance may not matter to you. Enterprises are also interested in sysadmins who have experience using with Hypervisors.
On my home server, I recently removed my two VirtualBox VMs and replaced them with two KVM VMs. The load average is significantly less with KVM, but I can't provide any objective data for you.