  We have analyzed XMM-Newton, Chandra, and Suzaku observations of Kepler's
supernova remnant (SNR) to investigate the properties of both the SN ejecta and
the circumstellar medium (CSM). For comparison, we have also analyzed two
similarly-aged, ejecta-dominated SNRs: Tycho's SNR, thought to be the remnant
of a typical Type Ia SN, and SNR 0509-67.5 in the Large Magellanic Cloud,
thought to be the remnant of an overluminous Type Ia SN. By simply comparing
the X-ray spectra, we find that line intensity ratios of iron-group elements
(IGE) to intermediate-mass elements (IME) for Kepler's SNR and SNR 0509-67.5
are much higher than those for Tycho's SNR. We therefore argue that Kepler is
the product of an overluminous Type Ia SN. This inference is supported by our
spectral modeling, which reveals the IGE and IME masses respectively to be
~0.95 M_sun and ~0.12 M_sun (Kepler's SNR), ~0.75 M_sun and ~0.34 M_sun (SNR
0509-67.5), and ~0.35 M_sun and ~0.70 M_sun (Tycho's SNR). We find that the CSM
component in Kepler's SNR consists of tenuous diffuse gas (~0.3 M_sun) present
throughout the entire remnant, plus dense knots (~0.035 M_sun). Both of these
components have an elevated N abundance (N/H ~ 4 times the solar value),
suggesting that they originate from CNO-processed material from the progenitor
system. The mass of the diffuse CSM allows us to infer the pre-SN mass-loss
rate to be ~1.5e-5 (V_w/10 km/s) M_sun/yr, in general agreement with results
from recent hydrodynamical simulations. Since the dense knots have slow proper
motions and relatively small ionization timescales, they were likely located a
few pc away from the progenitor system. Therefore, we argue that Kepler's SN
was an overluminous event that started to interact with massive CSM a few
hundred years after the explosion. This supports the possible link between
overluminous SNe and the so-called "Ia-CSM" SNe.
