the photoelectric effect is based upon the conservation of

When a photon of incident radiation collides with a free electron at the surface of target metal. The electron absorbs the entire energy of the photon. A part of this energy is used up by the electron to overcome the electrostatic force of attraction of the surface on the electron ,(This is called work function of the surface) and the balance amount of energy appears as the kinetic energy of the ejected electron  (This is the maximum possible kinetic energy)