  We present an optical and near-infrared photometric and spectroscopic study
of supernova (SN) 2009kn spanning ~1.5 yr from the discovery. The optical
spectra are dominated by the narrow (full width at half-maximum ~1000 km s^-1)
Balmer lines distinctive of a Type IIn SN with P Cygni profiles. Contrarily,
the photometric evolution resembles more that of a Type IIP SN with a large
drop in luminosity at the end of the plateau phase. These characteristics are
similar to those of SN 1994W, whose nature has been explained with two
different models with different approaches. The well-sampled data set on SN
2009kn offers the possibility to test these models, in the case of both SN
2009kn and SN 1994W. We associate the narrow P Cygni lines with a swept-up
shell composed of circumstellar matter and SN ejecta. The broad emission line
wings, seen during the plateau phase, arise from internal electron scattering
in this shell. The slope of the light curve after the post-plateau drop is
fairly consistent with that expected from the radioactive decay of 56Co,
suggesting an SN origin for SN 2009kn. Assuming radioactivity to be the main
source powering the light curve of SN 2009kn in the tail phase, we infer an
upper limit for 56Ni mass of 0.023 M_sun. This is significantly higher than
that estimated for SN 1994W, which also showed a much steeper decline of the
light curve after the post-plateau drop. We also observe late-time
near-infrared emission which most likely arises from newly formed dust produced
by SN 2009kn. As with SN 1994W, no broad lines are observed in the spectra of
SN 2009kn, not even in the late-time tail phase.
