What is an HRV unit? H stands for heat, R stands for Recovery of the heat in the extract air and U for unit. An HRV system is a ventilation system that takes advantage of the heat in the outgoing, used-up, ventilation air. The heat recovery allows the cold incoming air to be heated in the unit (heat exchanger) and then supplied to the rooms at near room temperature. This way you save energy (money) while providing a better indoor climate. A balanced ventilation system also has efficient filters that filters out pollen, dust, etc. which otherwise would enter into your home with the outdoor air.
An HRV unit is also often referred to as: ventilation unit, heat exchanger, air handling unit, residential air handling unit etc. All of these names refer to heat exchange in an HRV system. This should not be confused with a heat pump, that only works with the extract air.