Docker vs. Virtual Machines (VMs)

What’s the Real Difference?

If you’ve spent any time working in development, DevOps, or IT, you’ve probably heard the terms Docker and Virtual Machine tossed around like they’re interchangeable. Spoiler: They’re not. They both help you run apps in isolated environments, but how they do it is completely different.

.NET-Docker Series: Part 2

This is the second article in my .NET - Docker series, where we're exploring how to containerize, deploy, and run .NET applications using Docker.

In Part 1: Docker: Install, Run, and Test Your First Container, we got Docker up and running, from installation to running your first container. I hope you gave it a spin!

Now in Part 2, we’re stepping back to cover the basics: Docker vs Virtual Machines, what’s the difference.

Let’s break it down.

First off, what’s a Virtual Machine?

A Virtual Machine (VM) is basically a full-blown computer running inside another computer. It comes with its own operating system, virtualized hardware, and acts like an independent system, even though it’s running on top of another OS via something called a hypervisor.

So, if you’re using a Mac, you can spin up a VM that runs Windows or Linux. Each VM gets its own slice of CPU, memory, and storage.

1. Pros
   • Great isolation: Apps in one VM won’t mess with anything in another.
   • You can run different operating systems on the same physical machine.
   • Solid for legacy apps or stuff that really needs its own environment.
2. Cons
   • Heavy: Each VM has its own OS, so they eat up a lot of resources.
   • Slower to start and run.

Now, what’s Docker?

Docker is a containerization platform. Instead of virtualizing hardware, it virtualizes the operating system. That means you can package an app with everything it needs (code, libraries, dependencies) into a container, and run it anywhere Docker is installed.

Containers share the host OS kernel, which makes them super lightweight and fast compared to VMs.

1. Pros
   • Fast and efficient, containers start in seconds.
   • Use way fewer resources than VMs.
   • Super portable: run the same container on your laptop, in the cloud, or on a server.
2. Cons
   • Less isolation: Containers share the host OS, which can be a security concern if not managed right.
   • Can’t run a different OS inside a container (e.g., no Windows container on a Linux host).
     
     Image 1. 50000 feet view of VM and Docker

Can someone explain this image?
 

On the Left: Virtual Machines

In the VM stack, each application lives inside its own virtual machine, which includes.

• The App itself
• Its own libraries and binaries
• A full Guest OS

All of that is layered on top of a hypervisor, which runs on top of the Host OS.

This means every VM is carrying its own operating system along for the ride. That’s why VMs are more resource-heavy they’re like full mini-computers.

On the Right: Docker

In contrast, Docker containers strip all that down. Each container includes.

• The App
• Just the libraries and dependencies it needs.

And instead of each container running its own OS, they all sit on top of a shared Docker Engine, which talks directly to the Host OS.

The result? Less overhead, faster startup, and way better resource usage. You're not spinning up full OS environments, just isolated containers for your app.

So, Docker vs. VM—what’s the bottom line?

Here’s a quick side-by-side to sum it up.

Feature	Docker	Virtual Machine
Isolation	Process-level	Full OS-level
Startup Time	Seconds	Minutes
Resource Use	Low	High
Portability	Very high	Moderate
Security	Decent (less than VMs)	Strong
Use Case	Microservices, CI/CD, modern apps	Legacy apps, OS-specific tasks

So Which One Should You Use?

• Go with Docker if you want speed and efficiency and are working with modern app development.
• Stick with VMs if you need strong isolation, are dealing with legacy software, or need to run multiple OS types on one system.

Use Docker when you want speed and efficiency. Use VMs when you need full isolation and flexibility with operating systems.

Final Thoughts

At the end of the day, Docker and Virtual Machines are just different tools built for different jobs.

If you're building modern apps, want fast deployment, and need something lightweight, Docker is probably your best bet. It’s built for speed and scale. On the flip side, if you're dealing with legacy systems, need strong isolation, or have to run multiple operating systems on one machine, Virtual Machines still shine.

It’s not about which one is better, it’s about using the right tool for the job. And now that you know the difference, you’ll know exactly when to reach for each.