klystron can act as both an amplifier and oscillator whereas a
reflex klystron can act as only an oscillator.
klystron needs a buncher cavity(sometimes multiple bunchers) and
a catcher cavity whereas a reflex klystron needs only one
cavity.
klystron bunches electrons in forward direction, whereas the
other bunches in the reverse direction using a reflector plate.
klystron needs i/p signal (accelerating or deccelerating
potential) whereas the other dont.