Microsoft has released SP1 for Windows Server 2008 R2 and 7. One of the coolest feature Hyper-V got is the new Dynamic Memory allocation. This feature offers organizations to maximize thier use of memory resources by creating a memory pool that can be dynamically shared between virtual machines.

Do not be mistaken between this and Vmware memory overcommit, Dynamic memory sets a base start for memory and a maximum dynamic memory. The machine will start with the static memory always, but Hyper-V will be able to assign additional memory resource from the pool as needed by the machine and under the limit set on the max tab.

Here is where you can see the change

Dynamic Memory is further explained at the Virtualization Team BLOG