What does kV mean? Simply put, kV is just a measure of the speed with which the motor turns. (Less simply put, kV is the “unloaded RPM of the motor at 1 volt.”)
Specs for a typical brushless motor
Let’s look at a 1000kV motor. Its rating (1000kV) means that it will spin at 1000 RPM per volt. A 3-cell LiPo battery has a nominal voltage of 11.1V, so our 1000kV motor on a 3-cell will spin at 1000*11.1 or 11,100 RPM. Theoretically. Without any load. If you believe the manufacturer’s numbers.
In reality, the manufacturer’s numbers may be a bit … optimistic. And in reality that motor’s not running without any load. It’s got a propeller on there, and that propeller is pushing air, and that creates a load. It’s the difference between mashing the accelerator pedal on your car with the transmission in neutral vs. with it in gear. The motor acts very differently when it’s trying to pull thousands of pounds of metal down the street.
Still, kV numbers are useful for comparisons between motors. A 1600kV motor will spin roughly twice fast as an 800kV motor, and roughly half as fast as a 3200kV motor.
Motors of the same size and weight have roughly similar amounts of power. Consider two one ounce motors, one a 1200kV and the other a 3000kV. The two motors have about the same amount of power, but one spins much faster than the other. The 1200kV motor can spin a much larger propeller, moving larger volumes of air at slower speeds. The 3000kV motor is moving air at very high speeds. If you put the same large propeller on it as you use on the 1200kV motor, you’ll burn it up; it will be trying to move too much air, too quickly. (The load will be around 2.5 times as high at 3000kV as at 1200kV with the same prop.) A 3000kV motor will require a smaller prop, to move smaller volumes of air at high speed.
Very, very generally, kV ratings correspond to the speed at which the airplane is intended to fly. A screaming fast flying wing will probably be running a high kV motor, at least 1700kV and maybe as high as 3000kV to 3500kV. A slow-flying park flyer might be running something closer to 1000kV.