What is virtualisation, and what is the difference between a virtual machine and a container?

Virtualisation runs multiple isolated systems on one physical host through a hypervisor, which allocates CPU, memory, storage, and network to each guest. Type 1 hypervisors — ESXi, Hyper-V, KVM — run directly on hardware; Type 2 run on a host operating system.

The operational benefits are consolidation (better hardware use), isolation, snapshots for safe rollback, and live migration allowing hardware maintenance with no downtime.

Virtual machines versus containers:

• A VM virtualises hardware. Each has its own complete guest operating system, so it is heavy — gigabytes, and tens of seconds to boot — but strongly isolated and able to run any OS.
• A container virtualises the operating system. Containers share the host kernel and package only the application and its dependencies, so they are megabytes and start in under a second. Isolation is process-level, which is weaker than a hypervisor boundary.

How to choose: VMs for running different operating systems, for legacy applications, and where strong isolation is a security requirement — such as multi-tenant workloads. Containers for application deployment, microservices, and anywhere consistency between development and production matters.

Note: They are complementary rather than competing. In practice containers usually run on VMs — a Kubernetes cluster is typically a set of virtual machines — combining the isolation of one with the density and portability of the other.