A host operating system (host OS) is the operating system that provides the environment for other software. In hosted virtualization, it is the OS on which the hypervisor runs. The term also has a separate meaning in application virtualization, so its precise meaning depends on the architecture being discussed.
What does “host OS” mean in virtualization?
For hosted virtualization, NIST defines a host operating system as “the OS that the hypervisor runs on top of.” In this arrangement, the host OS is already running on the computer, and the hypervisor runs within that environment. The definition is sourced to NIST SP 800-125.
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This is a role in a software stack, not the name of a particular product. A host OS may be Windows, Linux, or another operating system; the term itself does not specify one.
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Does every hypervisor run on a host OS?
No. The host-OS definition applies to a hosted virtualization arrangement. Oracle’s VirtualBox documentation says VirtualBox requires an existing operating system, whereas a bare-metal, or type 1, hypervisor runs directly on the hardware. In the latter arrangement, describing the hypervisor as running on top of a conventional host OS would be inaccurate. See Oracle’s VirtualBox documentation.
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How is a host OS different from the operating system in a virtual machine?
A virtual machine runs a complete operating system, including its own kernel. The host OS, in a hosted setup, is the operating system beneath the hypervisor. They occupy different places in the stack: the guest OS belongs to the virtual machine, while the host OS provides the environment in which the hypervisor runs.
How does the term apply to containers?
Containers use the host OS kernel rather than running a complete operating system with a separate kernel for each container. They receive isolated views of system resources, and their user-mode system files are supplied through a base image. Microsoft explains this distinction in its container overview.
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Container-specific host operating systems
Some host operating systems are deliberately minimal and designed specifically to run containers. NIST describes this category as a container-specific operating system: a minimalistic host OS explicitly designed for that purpose. This is a design focus, not a different meaning of host OS. See the NIST glossary entry.
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Yes. In application virtualization, NIST uses “host OS” for the operating system kernel shared by multiple applications within that architecture. This is distinct from the hosted-hypervisor meaning and is sourced to NIST SP 800-190. When the context is unclear, specify whether you mean a hypervisor host or an application-virtualization host.
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