Definition

A hypervisor or virtual machine monitor (VMM) is a software that creates and runs virtual machines (VMs).

The physical hardware, when used as a hypervisor, is called the host, while the many VMs that use its resources are guests.

The hypervisor treats resources—like CPU, memory, and storage—as a pool that can be easily reallocated between existing guests or to new virtual machines.

It only handles the scheduling of processes, so hardware of host still executes the hardware functions of the guests.

Multiple different operating systems can run alongside each other and share the same virtualised hardware resources with a hypervisor. This is a key benefit of Virtualisation.

Types of Hypervisors

Type 1

Also known as native or bare metal hypervisors run directly on the host's hardware to manage guest operating systems.

It replaces the host OS and VM resources are scheduled directly to hardware by the hypervisor.

This is the most common type in enterprise data centres or other server-based environments.

KVM, Microsoft Hyper-V, and VMware vShpere are examples of type 1. KVM was merged into Linux Kernel since 2007.

Type 2

Also known as a hosted hypervisor, and is run on a conventional OS as a software layer or application.

It abstracts guest OS from host OS, VM resources are then scheduled agains a host OS, which is they executed against the hardware.

Better for users who want to run multiple OSes on a personal computer.

VMware Workstation and Oracle VirtualBox are examples of type 2.

Containers vs. VMs

Both are packaged computing environments that combine various IT components and isolate them from the rest of the system.

Hyper visor also allows running multiple OSs in VMs, but containers only run single type of OS. Container running on a Linux server can only run a Linux based OS.

Containers are sometimes thought of as a replacement for hypervisors, though it isn't accurate definition.

Container

Set of 1 or more processes that are isolated from the rest of the system.

Containers only allow processes to access the resources requested specifically.

Resource limits ensure that the container is able to run on a node that has enough capacity.

VM

Contain their own OS, to allow performing multiple resource-intensive functions at once.

Increased resources enables abstraction, splitting, duplication and emulation of entire servers, OSs, desktops, databases, and networks.

Security Considerations

VM provides an environment that is isolated from the rest of the system. If it is compromised, the system it runs on isn't impacted.

However, if hypervisor itself is hacked then it can cause problems for all of the VMs managed by the hypervisor.