I haven't used Hyper-V, but have run various versions (from 4.2.x through 7.x) of JBoss AS in VirtualBox and VMWare, in both Windows and Linux VMs, both 32-bit and 64-bit, and haven't run into any issues (but then, I haven't done any extensive performance testing in those environments either). You will need to do some performance testing and profiling to figure out what the slowdown is. And you might have to do that for both the VM and for the server on which it is running, so you might need some Hyper-V performance tuning expertise.
Looks like you are running JVM 1.6.0_29 (that's what I'm runnign and it also yields build number 20.4-b02), which is reasonably up to date. Some of people i work with have noticed issues with this and reverted to 1.6.0_26, but I don't think those were performance-related issues (I think they were network-related).