What Is a Hypervisor?
The Hypervisor's Job in a Virtualized Server
What is a hypervisor? It is the software that creates, runs and manages virtual machines on a physical host. It sits between the hardware and the guest operating systems, and it decides how the host's resources are divided among them.
Without a hypervisor, a physical machine can run only the operating system installed on it. The hypervisor changes that by presenting each guest with its own set of virtual hardware: a virtual processor, memory, disks and network adapters. Each guest then installs and runs its own operating system as if it were on a separate computer.
The hypervisor also keeps guests apart. It enforces boundaries so that one guest should not be able to read or change another guest's memory or disks.
How a Hypervisor Allocates CPU, Memory, Disk and Network
CPU. The hypervisor gives each guest virtual processors and schedules them onto the physical cores. If one guest is idle, its time can go to another guest that needs it.
Memory. Each guest believes it owns a continuous block of memory. The hypervisor maps those blocks onto real memory and can adjust the amounts as demand changes.
Disk. Guests usually see a single virtual disk. In reality, the hypervisor stores it as a file or a block device on the host's storage.
Network. Guests connect through virtual network adapters to a virtual switch, which the hypervisor links to the host's physical network.
Common Hypervisor Products and Where They Are Used
Several hypervisors are widely used, and each fits different situations. VMware ESXi is common in enterprise data centers. Microsoft Hyper-V is built into Windows Server and is often used for Windows workloads. KVM is part of the Linux kernel and is a common base for VPS hosting. Xen has a long history in cloud and hosting environments.
These products are named only as examples. They differ in management tools, supported hardware and licensing, so the right choice depends on the workload and the team that runs it. The KVM article explains one of them in more detail.
Frequently asked questions
Can a hypervisor run without a host operating system?
Yes, in some designs. Some hypervisors, such as VMware ESXi, run directly on the hardware without a general-purpose operating system underneath. Others run as an application on top of an existing OS. The two designs are compared in the type 1 vs type 2 hypervisor article.
Is a hypervisor the same as a virtual machine?
No. The hypervisor is the management software, while a virtual machine is one guest that the hypervisor runs. A single hypervisor can run many virtual machines at the same time, each with its own operating system and applications. For the basics of how guests see their hardware, read the virtualization basics article.