Software that resides in and virtualizes a user's computer. For an explanation of client virtualization, see definition #2 in desktop virtualization
Type 1 and Type 2
Type 1 hypervisors become the computer's master control program. These traditional virtual machine monitors (VMMs) create a separate subdivision (virtual machine) for each guest operating system.
Type 2 hypervisors use the operating system already installed in the computer and create a runtime environment for the guest operating system. See virtual machine monitor
Guest OS -> Hypervisor -------> Hardware
Guest OS -> Hypervisor -> OS -> Hardware