  We present an efficient method of inclusion of the core-valence correlations
into the configuration interaction (CI) calculations. These correlations take
place in the core area where the potential of external electrons is
approximately constant. A constant potential does not change the core electron
wave functions and Green's functions. Therefore, all operators describing
interaction of $M$ valence electrons and $N-M$ core electrons (the core part of
the Hartree-Fock Hamiltonian
  $V^{N-M}$, the correlation potential $\hat\Sigma_1({\bf r},{\bf r'},E)$ and
the screening of interaction between valence electrons by the core electrons
$\hat\Sigma_2$) may be calculated with all
  $M$ valence electrons removed. This allows one to avoid subtraction diagrams
which make accurate inclusion of the core-valence correlations for $M>2$
prohibitively complicated. Then the CI Hamiltonian for $M$ valence electrons is
calculated using orbitals in complete $V^{N}$ potential (the mean field
produced by all electrons); $\hat\Sigma_1$ + $\hat\Sigma_2$ are added to the CI
Hamiltonian to account for the core-valence correlations. We calculate
$\hat\Sigma_1$ and $\hat\Sigma_2$ using many-body perturbation theory in which
dominating classes of diagrams are included in all orders.
  We use neutral Xe I and all positive ions up to Xe VIII as a testing ground.
We found that the core electron density for all these systems is practically
the same. Therefore, we use the same $\hat\Sigma_1$ and $\hat\Sigma_2$ to build
the CI Hamiltonian in all these systems ($M=1,2,3,4,5,6,7,8$). Good agreement
with experiment for energy levels and Land\'{e} factors is demonstrated for all
cases from Xe I to Xe VIII.
