  We present a comprehensive analysis of the X-ray light curves of SN 1993J in
a nearby galaxy M81. This is the only supernova other than SN 1987A, which is
so extensively followed in the X-ray bands.
  Here we report on SN 1993J observations with the {\it Chandra} in the year
2005 and 2008, and Swift observations in 2005, 2006 and 2008. We combined these
observations with all available archival data of SN 1993J, which includes
ROSAT, ASCA, {\it Chandra}, and XMM-{\it Newton} observations from 1993 April
to 2006 August. In this paper we report the X-ray light curves of SN 1993J,
extending up to fifteen years, in the soft (0.3--2.4 keV), hard (2--8 keV) and
combined (0.3--8 keV) bands. The hard and soft-band fluxes decline at different
rates initially, but after about 5 years they both undergo a $t^{-1}$ decline.
The soft X-rays, which are initially low, start dominating after a few hundred
days. We interpret that most of the emission below 8 keV is coming from the
reverse shock which is radiative initially for around first 1000-2000 days and
then turn into adiabatic shock. Our hydrodynamic simulation also confirms the
reverse shock origin of the observed light curves. We also compare the
H$\alpha$ line luminosity of SN 1993J with its X-ray light curve and note that
the H$\alpha$ line luminosity has a fairly high fraction of the X-ray emission,
indicating presence of clumps in the emitting plasma.
