A Hypervisor is computer software, firmware or hardware that creates and runs virtual machines.

A computer on which a hypervisor runs 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. 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

Examples of Hypervisors:

  • Kernel-based Virtual Machine(KVM) - Used on various versions of Linux OS - Red Hat, Suse, Ubuntu etc.
  • VMWare - ESXi.
  • VMWare Workstation.
  • Oracle VM VirtualBox.
  • Xen / Citrix XenServer.
  • Microsoft Windows Server 2012 Hyper-V.
  • Another good example and easy to understand for network geeks is GNS3 tool that is installed on your windows/Linux machines. While you install GNS3 tool, it has a inbuilt hypervisor that helps you make your network typologies using various types of Cisco/Juniper routers, switches, firewalls that  virtually running via hypervisors The hypervisor helps these Virtual routers/switches use your PC/Laptop resources (RAM/CPU/HDD) on shared basis. Without a hypervisor a VM cannot utilize hardware resources of your Laptop/desktop machine.