The intel NUC is expensive, and Xen is hardware hungry if you get into larger setups. If you are looking to keep things relatively small then KVM on linux will keep hardware requirements lightweight, and Xen doesnt play nice with desktop methods of transferring files to the virtual guest, unless the files already reside on a network drive. comparatively getting KVM up and running is very easy. Also, if you were looking at transitioning running VMs from one server to another keep in mind you will essentially two copies of the same server. You cant transition a running VM from one processor architecture to another, and the specs will need to be close to similar. I have been using a workstation class PC I picked up for about $60 (xeon architecture) and I spent another $30 upgrading the four core processor to a 6 core processor that supports vt-d so if you want to get into hardware pass-though (off topic for this forum but may be relevant) it will work without running a critically insecure setup. If noise is not an issue you may look into used enterprise class servers, if you don't mind the sound of a jet engine at takeoff reverberating through your house.