A virtual machine is a cloud-based or on-premises based virtual computer which runs under the control of a hypervisor host or hypervisor cluster. There are two basic types of server virtualization, i.e. type 1 and type 2. Type 1 hypervisors provide install their own operating system (e.g. VMWare ESXi) and boot directly to that operating system. Type 2 hypervisors install themselves as server role on top of another operating system (e.g. HyperV server role). Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized hardware, software, or a combination.
A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called a host machine, and each virtual machine is called a guest machine. The hypervisor presents the guest operating systems with a virtual operating platform and manages the execution of the guest operating systems. Unlike an emulator, the guest executes most instructions on the native hardware. Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example, Linux, Windows, and macOS instances can all run on a single physical x86 machine. This contrasts with operating-system–level virtualization, where all instances (usually called containers) must share a single kernel, though the guest operating systems can differ in user space, such as different Linux distributions with the same kernel.
Different types of hardware virtualization include:
- Full virtualization – almost complete simulation of the actual hardware to allow software environments, including a guest operating system and its apps, to run unmodified.
- Paravirtualization – the guest apps are executed in their own isolated domains, as if they are running on a separate system, but a hardware environment is not simulated. Guest programs need to be specifically modified to run in this environment.