We have done quite a lot in Docker so far. But now let's explore the fundamental difference between Docker and a Virtual Machine.
When we are on any machine — whether Windows, Mac, or a host operating system — there is a kernel and above it is the application layer. On the application layer, all the apps in our system run (browser, etc.).
Docker virtualizes only the application layer, meaning it uses the host OS's kernel and only virtualizes the application layer.
A Virtual Machine virtualizes both the host OS kernel and the application layer.
| Feature | Docker | Virtual Machine |
|---|---|---|
| Virtualization Level | Application Layer Only | Application Layer + Operating System Kernel |
| Kernel Usage | Shares Host OS Kernel | Uses Its Own Guest OS Kernel |
| Size | Lightweight (MBs) | Heavyweight (GBs) |
| Startup Time | Very Fast (Seconds) | Slower (Minutes) |
| Resource Usage | Low | High |
| Performance | Near Native Performance | More Overhead |
| Portability | High | High |
| OS Compatibility | Depends on Host Kernel | Works Across Different Operating Systems |
| Isolation | Process-Level Isolation | Full OS Isolation |
Because Docker only virtualizes the application layer, Docker has less overhead — and this is why Docker is lightweight.
This also translates to Docker being faster and smaller in size compared to virtual machines, where sizes can be in GBs.
Virtual Machines are compatible with all underlying operating systems because VMs virtualize the host kernel itself — so it doesn't matter which machine we are running the VM on, the virtual machine uses its own kernel.
Docker was initially built for Linux-based systems. Because Docker virtualizes only the application layer and uses the host kernel, there can be compatibility issues with some non-Linux systems.
However, with Docker Desktop, this issue is largely resolved for Mac and Windows as well.