I couldn’t run the Windows Phone 8 Emulator on Windows 8.
When dual booting between Windows 7 and 8 (full install on HDD – no VHD) Hyper-V is getting into troubles a few times.
After removing it to clean things up, I got the error that a Hypervisor was already running so that I couldn’t enable the Hyper V windows feature anymore.
Settings the boot option “hypervisorlaunchtype” to Off in bcdedit for Windows 8 fixed that.
Oh, and disabling the virtual switch also helped (but removes internet access). Re-enabling the switch allows your internet access again and keeps the emulator running.
Good luck to all getting it up and running!