In the virtualization sphere, a Bare-Metal Hypervisor, also called Type 1 Hypervisor, is a virtualization software that gets installed on the host's bare hardware, hence the name. 

Bare-metal Hypervisors guarantee a greater efficiency and stability than Client Hypervisors (which are installed on top of operating systems), also called Type 2 Hypervisors, as they can directly manage computing resources withouth passing thru other softwares or operating systems.