Hyper-v is a microsoft product. That is your answer. It runs windows fine I'm sure, but it is your problem, not VM technology itself.

For example, I run bitcoin under Xen (linux/linux) and get khash/sec numbers that run with any I've seen posted.

Theoretically, I wouldn't think VM would affect bitcoin performance (coin mining at least) more than maybe a minute amount. The hashing code has zero system calls that need to be passed outside the virtual host, there is no overhead, especially in any good VM system like Xen that utilizes Thread Local Storage, Etc...

(not saying Xen is the best, only that bitcoin should run undetectably less good inside a VM as out; it does for me.)