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A hypervisor is an operating system instance, or a software package, that creates and manages virtual machines. The hypervisor typically runs on real hardware and allows multiple virtual machines to run on the same hardware. Virtual machines are also called guests.
Virtual machines run guest operating systems. A guest operating system may be different from the host operating system. For example, a Linux computer (hypervisor) may run Microsoft Windows 10 as a guest.
Modern hypervisors in the PC world include VMware ESX, VMware Player, Xen, Citrix XenServer, Microsoft Hyper-V, VirtualBox, and QEMU. VMWare Player, VirtualBox, and QEMU are application-layer hypervisors (essentially like normal programs), though they rely on some specialized operating system support for virtualization. Linux KVM and FreeBSD's bhyve are kernel modules that effectively convert a normal operating system into a hypervisor. In fact, it is often possible to log into a hypervisor (e.g., using SSH keys and use it like a normal operating system.
For more information, see virtualization software.