Samsung NX3000 has a
20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor . On the other hand, Panasonic GX850 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
One other difference between these two cameras that is worth mentioning is that Panasonic GX850'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 moiré occurring in certain scenes.
Below you can see the NX3000 and GX850 sensor size comparison.
As seen above, Samsung NX3000 has a 1.6x Larger sensor area than Panasonic GX850. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.