  The Kelvin-Helmholtz Instability (KHI) has been observed in the solar
atmosphere. Ion-neutral collisions may play a relevant role for the growth rate
and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar
prominences. Here, we investigate the linear phase of the KHI at an interface
between two partially ionized magnetized plasmas in the presence of a shear
flow. The effects of ion-neutral collisions and compressibility are included in
the analysis. We obtain the dispersion relation of the linear modes and perform
parametric studies of the unstable solutions. We find that in the
incompressible case the KHI is present for any velocity shear regardless the
value of the collision frequency. In the compressible case, the domain of
instability depends strongly on the plasma parameters, specially the collision
frequency and the density contrast. For high collision frequencies and low
density contrasts the KHI is present for super-Alfvenic velocity shear only.
For high density contrasts the threshold velocity shear can be reduced to
sub-Alfvenic values. For the particular case of turbulent plumes in
prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI
thanks to the ion-neutral coupling.
