Maybe nothing. Maybe more expensive motor parts. Maybe just a gimmick to sell even cheaper motors.
Almost all multicopters can go fairly fast. The better racers have better thrust to weight ratios and accelerate faster after corners.
KV is simply "RPMs per volt with no load (no prop)" so a 2000KV motor will produce 24000RPM at 12v (3 cell battery) where a 2500KV motor will produce 30000RPM at 12v. That same 2000KV motor will only produce 16000RPM when connected to an 8v (2 cell) battery. Generally if you change the KV or change the number of battery cells, you need a different prop for best performance and to avoid damaging via over propping. Under propping will never damage a standard brushless motor setup.
In the measurements I have made, a properly loaded (correct prop size) motor produces 20% to 33% less RPM than the KV indicates.
You can measure a motor's KV by putting two equally spaced pieces of black tape on the spinning part of the motor (assuming there is no label there that can confuse the tachometer). With no prop, run the motor to full power. Measure the RPM with a visual tachometer at the same time you measure the battery voltage (which will sag slightly under the load). KV=RPM/volts.