Here's the simple difference between AHU and HVAC:

• HVAC = Heating, Ventilation, and Air Conditioning
  It’s the whole system that controls temperature, humidity, and air quality in a building.
  HVAC includes everything — chillers, boilers, air conditioners, ducts, fans, thermostats, controls, and more.

• AHU = Air Handling Unit
  It’s one part of the HVAC system.
  The AHU's job is mainly to move and condition air — meaning it takes in outside air, filters it, heats or cools it (often with the help of a coil heat exchanger), and then distributes it through the building.

Simple way to remember:

• HVAC = the entire system (like a full orchestra).

• AHU = one important player (like the violin section).