  We have studied the electronic structure of multilayers composed of a band
insulator LaAlO$_3$ (LAO) and a Mott insulator LaVO$_3$ (LVO) by means of hard
x-ray photoemission spectroscopy, which has a probing depth as large as $\sim
60 {\AA}$. The Mott-Hubbard gap of LVO remained open at the interface,
indicating that the interface is insulating unlike the LaTiO$_3$/SrTiO$_3$
multilayers. We found that the valence of V in LVO were partially converted
from V$^{3+}$ to V$^{4+}$ only at the interface on the top side of the LVO
layer and that the amount of V$^{4+}$ increased with LVO layer thickness. We
suggest that the electronic reconstruction to eliminate the polarity
catastrophe inherent in the polar heterostructure is the origin of the highly
asymmetric valence change at the LVO/LAO interfaces.
