With Hyper-V R2, Microsoft include a new Processor Compatibility feature. Processor compatibility allows you to move a virtual machine up and down multiple processor generations from the same vendor.

Here's how it works:

When a Virtual Machine (VM) is started on a host, the hypervisor exposes the set of supported processor features available on the underlying hardware to the VM. This set of processor features are called guest visible processor features and are available to the VM until the VM is restarted.

When a VM is started with processor compatibility mode enabled, Hyper-V normalizes the processor feature set and only exposes guest visible processor features that are available on all Hyper-V enabled processors of the same processor architecture, i.e. AMD or Intel.  This allows the VM to be migrated to any hardware platform of the same processor architecture. Processor features are "hidden" by the hypervisor by intercepting a VM's CPUID instruction and clearing the returned bits corresponding to the hidden features.

Just so we're clear: this still means AMD<->AMD and Intel<->Intel. It does not mean you can Live Migrate between different processor vendors AMD<->Intel or vice versa.

In addition, you may be aware that both AMD and Intel have provided similar capabilities in hardware, Extended Migration and Flex Migration respectively. Extended and Flex Migration are cool technologies available on relatively recent processors, but this is a case where providing the solution in software allows us to be more flexible and provide this capability to older systems too. Processor Compatibility also makes it easier to upgrade to the newest server hardware. In addition, Hyper-V Processor Compatibility can be done on a per VM basis (it's a checkbox) and doesn't require any BIOS changes.