What is the difference between an emulator, a virtual machine and a container?
All three run software in an isolated environment, and they differ in how much they recreate — which determines their performance and what they can run.
An emulator simulates different hardware in software. It translates instructions intended for one processor architecture into instructions the host can execute.
Why you would: running software for hardware you do not have — a games console, an older architecture, an Android device on a PC.
The cost: it is slow. Every instruction must be interpreted or translated, which can be orders of magnitude slower than native execution. Techniques such as just-in-time recompilation reduce this substantially.
A virtual machine runs a complete operating system on virtualised hardware, managed by a hypervisor.
The crucial difference from emulation: a VM typically runs on the same architecture as the host, so instructions execute natively rather than being translated. Modern processors include hardware virtualisation support making this efficient.
What you get: a full OS with its own kernel, strong isolation, the ability to run a different operating system, and snapshots.
The cost: each VM includes an entire operating system, so it is measured in gigabytes and takes a minute or more to boot. Running many is resource-heavy.
A container packages an application with its dependencies but shares the host's kernel rather than including an operating system.
What follows: containers are much smaller — megabytes rather than gigabytes — and start in milliseconds, because there is no OS to boot. You can run hundreds where you could run a handful of VMs.
The trade-offs: containers must be compatible with the host kernel, so a Linux container needs a Linux kernel — which is why Docker on Windows or macOS quietly runs a Linux VM underneath. Isolation is also weaker than a VM's, since a shared kernel is a shared attack surface.
When to use which: emulator for different hardware; VM for a different OS or strong isolation; container for deploying and scaling applications consistently.