how to draw a plane in the centre of the hole or cylinder in solidworks?
Multiple ways to create it, here is one method:
I would make the sketch visible that you used for the cut. Then I would create a new plane, and constrain it to be parallel to the previous plane, and coincident with the centerpoint of the circle used to cut the cylinder. This uses the "Parallel plane at point" constraint.
1). Select "Plane" from "Reference Geometry" in "Features" Bar.
2). Click on cylinder one side cross section face.
3). Click on cylinder another side cross section face.
just check it that "plane" option is on or not in "View" manu.