  The exact NSVZ relation between a $\beta$-function of ${\cal N}=1$ SQED and
an anomalous dimension of the matter superfields is studied within the Slavnov
higher derivative regularization approach. It is shown that if the
renormalization group functions are defined in terms of the bare coupling
constant, this relation is always valid. In the renormalized theory the NSVZ
relation is obtained in the momentum subtraction scheme supplemented by a
special finite renormalization. Unlike the dimensional reduction, the higher
derivative regularization allows to fix this finite renormalization. This is
made by imposing the conditions $Z_3(\alpha,\mu=\Lambda)=1$ and
$Z(\alpha,\mu=\Lambda)=1$ on the renormalization constants of ${\cal N}=1$
SQED, where $\Lambda$ is a parameter in the higher derivative term. The results
are verified by the explicit three-loop calculation. In this approximation we
relate the $\bar{{DR}}$ scheme and the NSVZ scheme defined within the higher
derivative approach by the finite renormalization.
