  The physical nature of thermal composite supernova remnants (SNRs) remains
controversial. We have revisited the archival XMM-Newton and Chandra data of
the thermal composite SNR Kesteven 41 (Kes 41 or G337.8-0.1) and performed a
millimeter observation toward this source in the $^{12}$CO, $^{13}$CO, and
C$^{18}$O lines. The X-ray emission, mainly concentrated toward the
southwestern part of the SNR, is characterized by distinct S and Ar He-like
lines in the spectra. The X-ray spectra can be fitted with an absorbed
nonequilibrium ionization collisional plasma model at a temperature of 1.3-2.6
keV and an ionization timescale of 0.1-1.2$\times$10$^{12}$ cm$^{-3}$ s. The
metal species S and Ar are overabundant, with 1.2-2.7 and 1.3-3.8 solar
abundances, respectively, which strongly indicate the presence of a substantial
ejecta component in the X-ray-emitting plasma of this SNR. Kes 41 is found to
be associated with a giant molecular cloud (MC) at a systemic local standard of
rest velocity of -50 km s$^{-1}$ and confined in a cavity delineated by a
northern molecular shell, a western concave MC that features a discernible
shell, and an HI cloud seen toward the southeast of the SNR. The birth of the
SNR in a preexisting molecular cavity implies a mass of $\gtrsim$18 M$_{\odot}$
for the progenitor if it was not in a binary system. Thermal conduction and
cloudlet evaporation seem to be feasible mechanisms to interpret the X-ray
thermal composite morphology, and the scenario of gas reheating by the shock
reflected from the cavity wall is quantitatively consistent with the
observations. An updated list of thermal composite SNRs is also presented in
this paper.
