Microsoft Server 2008 Datacenter Edition
SELECT pricing is £201.68 per processor
Microsoft Schools Agreement (MSA)is £65.69 per processor each per annum
Therefore, a server with two QC Xeon E5504 2.00GHz would be two licenses so £403.36 SELECT or £131.38 per annum under an MSA.
This is six times the price of a standard server licence (you wouldn't use Enterprise on virtual servers). This means that it is no cheaper unless you are a) running more than six VMs and b) you don't have any existing licences for re-use (say from converted physical machines).
In addition, you would not have the XenCenter management as there are not many tools with Hyper-V.