  X-ray observations of middle-aged pulsars allow one to study nonthermal
radiation from pulsar magnetospheres and thermal radiation from neutron star
(NS) surfaces. In particular, from the analysis of thermal radiation one can
infer the surface temperatures and radii of NSs, which is important for
investigating evolution of these objects and constraining the equation of state
of the superdense matter in the NS interiors. Here we present results of XMM
observations of three middle-aged pulsars, J0538+2817, B0656+14 and J0633+1746
(Geminga), and briefly discuss mechanisms of their X-ray emission.
