The hypervisor, also known as the Virtual Machine Monitor( VMM), is the core of most server virtualization products. The VMM manager represents the virtualization layer that enables the simultaneous execution of several virtual machines( VM) as well as their control and in this capacity is also called host - as opposed to guest - or (misleadingly, since it can be confused with the operating system) host.
The layer model of the hypervisor architecture
The VMM or hypervisor layer allows several different operating systems to share the hardware resources of a computer by transparently dividing the available capacities of RAMs, processors, input/output (I/O), DMA controllers and all other relevant components among the guest systems.
Depending on the hypervisor architecture, the VMM manager runs directly on the hardware (hypervisor type 1). This hypervisor type, in which the virtual machine (VM) uses the resources provided by the hypervisor, is also known as a native hypervisor or bare-metal hypervisor. It is different for hypervisor applications when it provides the I/O resources of the network and storage systems within a normal (host) operating system. This hypervisor type 2 is also called hosted hypervisor. Some rely on features of the hardware or special virtualization features of the central processing unit( CPU) such as Intel Virtualization Technology (Intel VT) or AMD Virtualization (AMD-V) to run virtual machines.
Well-known representatives of the Type 1 hypervisor are: IBM z/VM, Xen, VMware ESX, Sun Logical Domains. Their architecture has the advantage of being very lean and very robust. At the same time, they are the most performant representatives of server virtualization. Examples of the Type 2 hypervisor: VMware Server + Workstation, Microsoft Virtual PC + Server, Qemu, Parallel Workstation + Desktop.