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Dynamic memory is a cool feature in Hyper-V R2 SP1 and let’s you increase the density of VM’s on your Hyper-V Server. In short, the Hyper-V Server can assign memory to VM’s that need it, but ‘take’ it from VM’s that do not need it.

It’s all about the workload of course. Exchange Server 2010 is fully supported to run on Hyper-V, except for the Dynamic Memory part. Exchange Server wants only one thing: allocate as much memory as possible for caching mailbox data. When using Dynamic Memory, Exchange will allocate more memory and the Hyper-V server will allow this. But as soon as the Hyper-V server needs to reclaim memory from the Exchange Server and the ‘balloon starts to inflate’ Exchange Server will suffer performance issues. So from an Exchange point of view it’s better to allocate a fixed amount of memory (plenty of memory) to prevent nasty surprises.

For Microsoft official support recommendations on Exchange Server you might want to check this page: Microsoft Support Policies and Recommendations for Exchange Servers in Hardware Virtualization Environments

And for the dynamic memory part scroll down to “Dynamic Memory Allocation Considerations”

Quote from this article:
“Many of the performance improvements in recent versions of Exchange are based on the efficient use of an appropriately-sized RAM allocation. This is particularly true of improvements that are related to reductions in I/O operations. The performance optimizations rely on Exchange caching data in RAM. When RAM is dynamically reduced, the expected performance of the system cannot be achieved. In this scenario, Exchange may exhibit reduced performance, or end-users may experience reduced performance when connecting to Exchange. Therefore, for virtual machines that are running Exchange in a production environment, it is best to turn off memory oversubscription or dynamic memory allocation. Instead, configure a static memory size that is based on the appropriate values for Exchange 2007.”

The same is true for Exchange Server 2010.