Hyper-V uses dynamic memory to address physical RAM limits. You assign the VM a minimum and a maximum amount of memory, and the VM will only use what it needs at that time.
Hyper-V does not share guest VM's memory pages with other guest VMs. I don't know if VMware does that or not, but that screams security breach to me.
Each guest OS/partition inside of Hyper-V however, still has its own memory manager and will still use memory management techniques (e.g. memory combining for Windows 8) that are unique to that operating system.
Edit: Also, it's probably not important to make the distinction between Hyper-V Server 2012 and regular Server 2012, since as far as I know there are no relevant differences between how Hyper-V works on those editions of Windows. Just licensing differences.
This feature of Windows 8 would also jive really well with the improvement in Hyper-V 2012 too, since Hyper-V can now reclaim memory from VMs over time. And it sounds like with memory combining, Windows 8 VMs would have more memory to give up after startup.