Parametric X-rays (PXR) along the velocity of relativistic electrons crossing a crystalline target is studied in this work. The detailed theory of PXR for Laue scattering geometry is developed with account of contributions of both PXR and transition radiation (TR) to the total emission yield. An influence of photoabsorption and interference between PXR and TR on the forward PXR properties are studied. Most appropriate conditions for the real experiment devoted to forward PXR observation are elucidated on the basis of the developed theory. The advantage of the use of heavy crystals with this aim in mind is shown. The expected experimental results are discussed.