A virtual machine (VM) is a software emulation of a physical server with an operating system.
From the applications point of view, the virtual machine provides the look and feel of a real physical machine including all its hardware components.
The virtualisation software that creates the VMs and performs the hardware virtualisation is known as the hypervisor.
Some examples of popular hypervisors are VMWare vSphere, Microsoft Hyper-V, Citrix XenServer, and Red Hat Kernel-based Virtual Machine.
These are two types of Hypervisor.
One type runs directly on the system hardware. It is commonly known as bare metal or native.
The other type requires a host operating system to run. This is the type of hypervisor that is used by client devices.
Virtual Machines can be migrated from one server to another whilst maintaining integrity during movement. This can allow virtual machines to be migrated from one server to another with zero down time.
Another advantage is high availability, if one hypervisor fails the virtual machines can be seamlessly migrated to another host.