Microsoft Support Policies and Recommendations for Exchange Servers in Hardware Virtualization Environments states that Exchange Server is supported as long as virtualization software is Windows Server 2008 with Hyper-V technology, Microsoft Hyper-V Server, or any third-party hypervisor that has been validated under the Windows Server Virtualization Validation Program.
One of the virtualization products that have passed the SVVP requirements for Windows Server 2008 is Citrix XenServer 5. If you want to know more details, Citrix has released a technical whitepaper: "Virtualizating Microsoft Exchange Server 2007 with Citrix XenServer".
There’s also more information available in the blog post Virtualize Exchange Server 2007 on Citrix XenServer? Absolutely!
In our technical whitepaper entitiled "Virtualizating Microsoft Exchange Server 2007 with Citrix XenServer", we demonstrated that Exchange Server virtualizes very well on XenServer. Not only did virtual Exchange Servers running on XenServer exceed Microsoft performance recommendations in every performance category, but in many cases virtual Exchange servers exceeded the perfomance of their native server counterparts! Here's an example of what I mean. This was taken from the latest tests we just completed using Exchange Server 2007 VMs with Windows Server 2008 on XenServer 5.0: