A theoretical study on the kinematic and dynamic approximations of parametric X-ray radiation (PXR) is presented. The results obtained are examined and compared. The ratio of the absolute maxima of kinematic and dynamic PXR can be obtained. The results are closer at lower energies and their correlation depends on the angle of incidence. These theoretical results indicate that in some energy regions and angle of incidence it may be necessary to apply the dynamic approximation of PXR for greater accuracy.