Each operating system appears to have the host’s processor, memory, and resources to itself. Instead, the hypervisor is controlling the host processor and resources, distributing what is needed to each operating system in turn and ensuring that the guest operating systems/virtual machines are unable to disrupt each other.
Hypervisors are classified into two types:
Bare Metal/Native Hypervisors— Software systems that run directly on the host’s software as a hardware control and guest operating system monitor. A guest operating system thus runs on another level above the hypervisor. This is the classic implementation of virtual machine architectures.
A variation of this is embedding the hypervisor in the firmware of the platform, as is done in the case of Hitachi’s Virtage hypervisor and VMware ESXi.
Embedded/Host Hypervisors— Software applications that run within a conventional operating system environment. Considering the hypervisor layer being a distinct software layer, guest operating systems thus run at the third level above the hardware.