How to fix "The virtual machine could not be started because the hypervisor is not running" on a desktop

If you get this error message on a desktop machine (not a server):

Most desktop machines have "Virtualization" Disabled by default in their BIOS. If you  have enabled Hyper-V on a desktop machine to test, as opposed to a server machine, you must ensure Virtualization is Enabled or turned on in the BIOS setting in order to run virtual machines.

When you enable the Hyper-V role in Windows Server, it automatically disables desktop features like Hibernation / Sleep.  

If, for some reason, such as testing, you must have these features enabled, you can change the 'Start' Value from 1 to 3 in  
Y_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\hvboot\Start = DWORD 3.However, before changing any registry values ensure that you read and understand http://support.microsoft.com/kb/136393 . Changing this value is not supported by Microsoft.

NOTE: This will cause the hypervisor to stop and when you try to run the virtual machine again you will get this error message:  
 "the virtual machine could not be started because the hypervisor is not running"
even though you have "Virtualization" Enabled in the BIOS setting.

• Since you have changed the Registry value for "hvboot" to enable Hibernation / Sleep feature,  you will need to start the Hypervisor manually each time.

Do this: Open Command Prompt window with Administrative privilege and run the following command > net start hvboot

Start the Virtual Machine

NOTE: The most likely reason for the error message, and the reccommended fixes for it, are given here: http://social.technet.microsoft.com/wiki/contents/articles/hyper-v-troubleshooting-event-id-3112-the-virtual-machine-could-not-be-started-because-the-hypervisor-is-not-running.aspx

If you get this message on a server, and the error started after making a change to the BCD store, see this blog post: Hypervisorlaunchtype not set after BCD changes – here is the fix