Why the performance decrease for an SSD when using Hyper-V?

I've just installed my new M4 256GB on a Windows Server 2008 R2 Hyper-V server.

On the host system using Crystal Disk Mark I've got 212 Mb/s as 4k QD32 write performance, but when I create a VHD disk on the SSD and test using the same configuration on a guest Windows Server 2008 R2, I get 7 Mb/s, i.e. about 30 times slower.

Any hint?

Of course Fixed size vhd