This seems to be common for some reason when trying to run Windows 7 under KVM and 7 has always had strange issues with KVM. What happens for me is after the install it simply won't boot, it stays on the POST screen and does nothing.
For some reason it appears the boot sector either doesn't get installed or gets corrupted with KVM.
The solution is to boot the install disc for Windows 7 and reinstall the windows MBR into the boot sector:
1. Boot the 7 install disc and choose the "Use recovery tools" option instead of installing.
2. Then click "Command Prompt" at the bottom and run the following:
bootsect /nt60 SYS /mbr
The end message should be something like "Bootcode was succesfully updated on all target volumes".
After that reboot and Windows 7 should load.