This work is devoted to the study of the point defect diffusion features in metals. In particular, we propose the model, which allows calculating activation volumes that describe the influence of pressure on the diffusion processes in solids. Our model realizes a new approach that makes it possible to self-consistently determine atomic structure near defect and constants characterizing the displacement of atoms in an elastic matrix around computational cell. Also we take into consideration that the energy of perfect system and system with a defect differently depends on the outer pressure, and this gives an addition to the values of migration and formation volumes. This addition can comprise a considerable part of activation volume. Moreover, we take into account that the atomic jump is a momentary process and so we carry out only partial relaxation of the atomic structure in the vicinity of a defect. The formation and migration energies and formation and migration volumes have been calculated for vacancies, di-vacancies and interstitials in bcc iron and tungsten using pair and many-body potentials.