Both Pentax KP and Pentax K-3 have 24.0 MP resolution sensors but Pentax KP's sensor is APS-C (23.5 x 15.6 mm ) and Pentax K-3's sensor is APS-C (23.5 x 15.6 mm ).
Since Pentax K-3's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Pentax KP.
One other difference between these two cameras that is worth mentioning is that Pentax K-3's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the KP and K-3 sensor size comparison.
Pentax KP and Pentax K-3 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.