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A virtual machine manager (VMM) is the software layer that creates virtual machines (VMs) and divides a physical computer’s processing power, memory and storage among them. The same phrase also names Microsoft System Center Virtual Machine Manager, a separate datacenter management product, so the meaning depends on the context where you found it.
What a virtual machine manager does
A virtual machine manager sits between physical hardware and the software that runs on it. Its core job is allocation. Red Hat’s “What is a hypervisor?” article, published January 3, 2023, puts it this way: a hypervisor “is software that pools computing resources—like processing, memory, and storage—and reallocates them among virtual machines (VMs).” The physical computer is called the host, and each VM running on it is called a guest.
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In practice, the VMM decides how much processor time, memory and storage each guest receives, creates or starts guests on request, and keeps them isolated from one another so that one guest’s operating system does not directly control another’s resources.
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Three terms, one layer
Documentation uses several names for the same virtualization layer:
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- Virtual machine manager (VMM) is the name used in the broadest sense for software that creates and manages VMs.
- Virtual machine monitor is an older and more academic term for the same control layer.
- Hypervisor is the most common term in vendor documentation today.
These terms are used interchangeably in general explanations, but individual products do not always define them identically. When a specific product’s manual uses one of these words, follow that product’s definition.
Type 1 and Type 2 hypervisors
Hypervisors are usually grouped by where the virtualization layer runs. VMware’s explanation of the bare-metal hypervisor and Red Hat’s overview both use this split. The useful comparison is not which product is better but where the layer sits relative to the hardware.
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| Comparison point | Type 1 (native or bare-metal) | Type 2 (hosted) |
|---|---|---|
| Where it runs | Directly on the host hardware | As a software layer on top of a conventional host operating system |
| How guest resources are handled | The virtualization layer allocates hardware resources to guests directly | Guest resources are scheduled through the host operating system |
| Typical setting described by vendors | Enterprise data centers and servers | Individual users running more than one operating system on a personal computer, and software testing |
Type 1: running on the hardware
A Type 1 hypervisor installs on the machine itself and boots before any general-purpose operating system. Because it has no host operating system between it and the hardware, it is the model vendors describe for server and data center workloads. Performance and security depend on the product, configuration and hardware, so neither should be assumed from the category alone.
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A Type 2 hypervisor is an application installed on an ordinary desktop or laptop operating system. You start it the way you start any other program, and it creates guests that run alongside your normal applications. This arrangement is convenient for development, testing and running a second operating system, but the host operating system takes part in every guest’s resource scheduling.
A virtual machine manager is not a virtual machine
The VMM and the VM are different things. The VMM is the control layer that manages or enables virtual machines. Each VM is a software-defined computer with its own virtual hardware and its own operating system, running on a physical host that the VMM controls. VMware’s explanation of virtual machines describes the VM as that self-contained software computer, separate from the software that runs it.
Microsoft System Center Virtual Machine Manager
The name collision causes most confusion. Microsoft System Center Virtual Machine Manager is a management product, not a generic name for a hypervisor. Microsoft’s documentation describes it as a unified management experience for datacenter components and as a tool for provisioning and managing resources for VMs and services. It manages Hyper-V and VMware hosts and clusters, and it is aimed at administrators who run many hosts rather than at a single hypervisor.
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Because the product manages hypervisors instead of being one, it does not replace Hyper-V or VMware ESXi. It sits above them. Microsoft’s documentation is hosted at Microsoft Learn: Virtual Machine Manager documentation.
How to tell which meaning applies
- If the text talks about creating guests, allocating CPU and memory, or choosing between bare-metal and hosted setups, it means the generic virtualization layer.
- If it talks about managing many hosts, clusters, or datacenter resources from a central console, it likely means System Center Virtual Machine Manager.
- If it names a version, such as a System Center release, check that release’s requirements before relying on any compatibility statement. Microsoft’s system requirements for Virtual Machine Manager page lists supported host operating systems and VMware versions, and those lists change between releases.
Where to read more
For a vendor-neutral starting point, VMware’s What is a Hypervisor? explains the Type 1 and Type 2 split. For the virtual machine itself, see VMware’s What is a Virtual Machine? page.
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