User mode emulation

User mode emulation refers to the ability of certain emulators to emulate userland code running on top of a specific operating system, usually Linux.

For example, QEMU allows you to run a variety of userland ELF programs directly on it, without an underlying Linux kernel running.

User mode emulation is achieved by implementing System calls and special filesystems such as /dev manually on the emulator one by one.

The general tradeoff is that simulation is less acurate as it may lack certain highly advanced kernel functionality you haven't implemented yet. But it is much easier to run executables with it, and you don't have to wait for boot to finish before running, you just run executables directly from the command line.