  We have performed an XMM-Newton imaging and spectroscopic study of supernova
remnant (SNR) W28, a prototype mixed-morphology or thermal composite SNR,
believed to be interacting with a molecular cloud. The observed hot X-ray
emitting plasma is characterized by low metal abundances, showing no evidence
of ejecta. The X-rays arising from the deformed northeast shell consist of a
thermal component with a temperature of $\sim0.3$ keV plus a hard component of
either thermal (temperature $\sim 0.6$ keV) or non-thermal (photon index
$=0.9$-2.4) origin. The X-ray emission in the SNR interior is blobby and the
corresponding spectra are best described as the emission from a cold
($kT\sim0.4$ keV) plasma in non-equilibrium ionization with an ionization
timescale of $\sim4.3\times 10^{11}$ cm$^{-3}$ s plus a hot ($kT \sim 0.8$ keV)
gas in collisional ionization equilibrium. Applying the two-temperature model
to the smaller central regions, we find non-uniform interstellar absorption,
temperature and density distribution, which indicates that the remnant is
evolving in a non-uniform environment with denser material in the east and
north. The cloudlet evaporation mechanism can essentially explain the
properties of the X-ray emission in the center and thermal conduction may also
play a role for length scales comparable to the remnant radius. A recombining
plasma model with an electron temperature of $\sim 0.6$ keV is also feasible
for describing the hot central gas with the recombination age of the gas
estimated at $\sim2.9\times 10^4$ yr.
