AirSim is a open-source, cross platform simulator for drones, built on Unreal Engine. It provides physically and visually realistic simulations of Pixhawk/PX4 using either Hardware-In-The-Loop (HITL) or Software-In-The-Loop (SITL).
The main entry point for the documentation is the Github AirSim README.
The instructions for HITL setup are here.
The walkthrough video below shows the setup for HITL with Pixhawk in more detail.
The instructions for SITL setup are here.