A. Different processor versions support different features. Because OSs or applications may check which features are available, a VM that's moved to a different host using Quick Migration or Live migration wouldn't be aware the features are missing, which could cause the application to crash. That's why you can't perform Quick Migrations or Live Migrations between hosts with different processor versions.
Windows Server 2008 R2 introduced a processor compatibility option that's set on a per-VM basis. This option reduces the set of processor features to a fixed set, regardless of what's actually supported on the processor. The VM will only see a common set of features, no matter which host it runs on. This exposed set of features isn't generated based on the lowest common set of supported features across all the hosts—it's a fixed set.
The following features are hidden on AMD processors when processor compatibility is enabled:
SSSE3, SSE4.1, SSE4.A, SSE5, POPCNT, LZCNT, Misaligned SSE, AMD 3DNow!, Extended AMD 3DNow!
On Intel processors, these features are hidden:
SSSE3, SSE4.1, SSE4.2, POPCNT, Misaligned SSE, XSAVE, AVX
Note that even with processor compatibility, you can't mix Intel and AMD processors in the same cluster.