There are two models which describe the origin of ``parametric x-ray radiation'' (PXR): the dynamical (``quasi-erenkov'') model and the kinematical model. A theory is developed that unifies the two models. An analytic expression is obtained which gives a certain dynamical effect at high energies while reduced to the kinematical formula at low energies. A Green function analysis cleary shows that dynamical effect is not essential for the description of PXR.