guest OS (guest operating system)

What is a guest operating system (guest OS)?

A guest operating system is the operating system installed on either a virtual machine (VM) or partitioned disk. It is usually different from the host operating system. Simply put, a host OS runs on hardware, while a guest OS runs on a VM.

Virtualization and guest OS

Virtualization technology allows a single computer to run more than one OS at the same time. Thus, a single physical machine can be configured as multiple VMs. These VMs are isolated sections of hardware with storage, processing, memory and network capacity. Virtualization also allows resource sharing between the host OS and guest OS.

diagram showing guest OS, virtual machine and host operating system
A guest OS is installed inside a virtual machine, which is installed on the host operating system that runs a server. Guest OSes are often different from the host OS.

A guest OS provides an additional OS for applications and is required before a VM can be deployed. It can be used for testing by developers without impacting anything outside that VM, such as the data already in production use. An example of a guest OS would be Windows Server 2022 in a VM created by the VMware ESXi hypervisor. Another example would be Boot Camp, which allows Mac users to run a Windows OS as the guest OS within a VM on their Mac.

A guest OS on a VM can be different from the host OS. But when the guest is deployed on a partitioned disk, the guest OS must be the same as the host OS. So, if the host is running Windows, then any guest OS on a partitioned disk must also run Windows.

How does a guest OS differ from a host OS?

A guest OS is installed on a VM, while the host OS is installed on and runs the computer (host). The host OS also interacts with the underlying hardware. There can be multiple guest operating systems running on a computer, while the host is usually restricted to one OS.

A physical server can have multiple VMs, with each VM running its own guest OS. One guest OS could be Windows, while another could be Ubuntu or Linux, for example. The guest OS and host OS are separate and operate independently of each other. They can run simultaneously, although the host OS must be started first.

A Type 1 hypervisor (bare-metal hypervisor) can create VMs, which can run guest OSes. In other words, the guest OS is delivered in a VM environment through a hypervisor. The host OS will run on the host machine and the guest OS operates within it, which can limit file saving and other operations in the VM.

Advantages of a guest OS

Since a guest OS operates independently of the host OS, one of its significant advantages is that it allows programs or applications to run that are not compatible with, or cannot run in the host OS. A guest OS features a "lean" build which helps alleviate memory and other system requirements in a virtualization environment.

Additionally, a guest OS comes in handy to test programs or applications, and run them if they require different operating systems, on the same machine or hardware.

Installing a guest OS through an ISO image

There are several ways to install a guest OS on a virtual machine, including the following:

  • Installer disc: CD-ROM or DVD-ROM;
  • ISO image file, an OS file stored on an optical disc; and
  • PXE server, a preboot execution environment downloaded from the network.

Explore the difference between Type 1 and Type 2 hypervisors and what to know for your virtualized storage selection process. Also, see how microservices and containers work apart and together, how to design your systems with virtualization architecture in mind and some examples of embedded hypervisors.

This was last updated in January 2022

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