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 The following are the supported scenarios for Exchange 2010 SP1 :

• The Unified Messaging server role is supported in a virtualized environment.
• Combined Exchange 2010 high availability solutions (database availability groups (DAGs)) with hypervisor-based clustering, high availability, or migration solutions that will move or automatically failover mailbox servers that are members of a DAG between clustered root servers

HyperV Guest Configuration

Keep in mind that because there are no routines within Exchange Server that test for a virtualized platform, Exchange Server behaves no differently programmatically on a virtualized platform than it does on a physical platform.

Determining Exchange Server Role Virtual Machine Locations

When determining Exchange Server Role virtual machine locations, consider the following general best practices:

• Deploy the same Exchange roles across multiple physical server roots (to allow for load balancing and high availability).
• Never deploy Mailbox servers that are members of the same Database Availability Groups (DAGs) on the same root.
• Never deploy all the Client Access Servers on the same root.
• Never deploy all the Hub Transport servers on the same root.
• Determine the workload requirements for each server and balance the workload across the HyperV guest virtual machines.

Guest Storage

Each Exchange guest virtual machine must be allocated sufficient storage space on the root virtual machine for the fixed disk that contains the guest’s operating system, any temporary memory storage files in use, and related virtual machine files that are hosted on the root machine.Consider the following best practices when configuring Hyper-V guests:

• Fixed VHDs are recommended for the virtual operating system.
• Allow for a minimum of a 15-GB disk for the operating system, allow additional space for the paging file, management software, and crash recovery (dump) files. Then add Exchange server role space requirements.
• Storage used by Exchange should be hosted in disk spindles that are separate from the storage that hosts the guest virtual machine’s operating system.
• For Hub Transport servers, correctly provision the necessary disk space needed for the message queue database, and logging operations.
• For Mailbox servers, correctly provision the necessary disk space for databases, transaction logs, the content index, and other logging operations. .

Guest Memory : Dynamic Memory should be disabled
Memory must be sized for guest virtual machines using the same methods as physical computer deployments. Exchange—like many server applications that have optimizations for performance that involve caching of data in memory—is susceptible to poor system performance and an unacceptable client experience if it doesn’t have full control over the memory allocated to the physical computer or virtual machine on which it is running.
Many of the performance gains in recent versions of Exchange, especially those related to reduction in input/output (I/O) are based on highly efficient usage of large amounts of memory. When that memory is no longer available, the expected performance of the system can’t be achieved. For this reason, memory oversubscription or dynamic adjustment of virtual machine memory must be disabled for production Exchange servers.

Deployment Recommendations

When designing an Exchange Server 2010 virtualized environment, the core Exchange design principles apply. The environment must be designed for the correct performance, reliability, and capacity requirements. Design considerations such as examining usage profiles, message profiles, and so on must still be taken into account.

See this article (Mailbox Storage Design Process) as a starting point when considering a high availability solution that uses DAGs.

Because virtualization provides the flexibility to make changes to the design of the environment later, some organizations might be tempted to spend less time on their design at the outset. As a best practice, spend adequate time designing the environment to avoid pitfalls later.

Group the Exchange Server roles in such a way that balances workloads on the root servers. Mixing both roles on the same HyperV root server can balance the workloads and prevent one physical resource from being unduly stressed, rather than if the same roles were put on the same hosts

The updated support guidance applies to any hardware virtualization vendor participating in the Windows Server Virtualization Validation Program (SVVP).

Best Practices for Virtualizing Exchange Server 2010 with Windows Server 2008 R2 Hyper-Vwhitepaper. This whitepaper is designed to provide technical guidance on Exchange server roles, capacity planning, sizing and performance, as well as high availability best practices.

Complete system requirements for Exchange Server 2010 running under hardware virtualization software can be found in Exchange 2010 System Requirements. Also, the support policy for Microsoft software running in non-Microsoft hardware virtualization software can be found here.