Just recently setup a new machine with Windows 8 and decided to install Hyper-V so that I could run windows XP for some of my legacy applications that will not run under Windows 8 64 bit. After installing Hyper-V components I attempted to setup a new virtual machine and start the install process when I was faced with the following error: "could not be started because the hypervisor is not running"
It turned out that I needed to run the following command
bcdedit /set hypervisorlaunchtype auto
After running the above command and restarting my machine it now works. Hope this helps save someone else hours they cannot back due to this issue.