Windows Server 2012 Essentials doesn't have Hyper-V support. So the short answer is that you can't; at least not with the built-in tools.:|
I don't have any direct experience here, but I think the only way you could pull this off is to use Microsoft's Hyper-V server package, and then install both 2012E and Win8 as guests.
Can try installing win8 in virtualbox on server 2012 =P
not sure if virtualbox supports win8 yet tho...
