A hypervisor (or virtual machine monitor, VMM, virtualizer) is a kind of emulator; it is 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. 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.