The preferences screen of the PartDesign are found in the Preferences window (Menu Edit → Preferences → PartDesign).
In order to display an object efficiently the surface is tesselated, i.e. it is displayed with some tiny deviations. This tesselation applies not only to PartDesign models, but also to other objects. There is a lower limit for the tesselation of 0.01%. If you really want to spend the additional time you can reduce the lower limit even further. Open Tools->Edit parameters... This opens the parameter editor where you navigate to BaseApp->Preferences->Mod->Part. Right click in the right portion of the Parameter Editor window and from the pop-up menu choose "New float item". Set the value to the minimum tesselation of your choice. Please keep in mind that the value in the preferences section is shown in %, i.e. for a value of 0.005% you have to enter "0.00005". The smallest value possible is 1e-7.