A little while ago we announced that - in order to help people run Hyper-V Containers on their existing infrastructure - we would be adding support for nested Hyper-V.
Well - yesterday we flighted the first build with this capability in it:
Now, this is very early functionality - and there are plenty of caveats. You can read all about it here: http://blogs.technet.com/b/virtualization/archive/2015/10/13/windows-insider-preview-nested-virtualization.aspx
Cheers,
Ben