The kinetic energy of the electron is transformed into electromagnetic energy. The function of the x-ray machine is to provide a sufficient intensity of electron flow from the cathode to anode in a controlled manner.
Kinetic energy is the energy of motion.
In determining the magnitude of the kinetic energy of a projectile, the velocity is more important than the mass. In a x-ray tube, the projectile is the electron. The x-ray machine is a remarkable instrument. It conveys to the target an enormous number of electrons at a precisely controlled kinetic energy. The distance between the filament and the target is only about 1 to 3 cm.