I know the difference between the two:
Passive jammers claim to jam radar by receiving the inbound microwave beam, mixing it with some white noise, then reflecting the altered signal back to the radar, so confusing it that no speed is displayed.
Active jammers transmit a signal designed to make the radar disregard its own return signal, placing it on an endless loop as it continuously tries to aquire a speed.
But are there any active jammers that are sold? Or all they all passive?