vimdiff file1 file2
besides differences shows also same lines from both files. Is it possible to hide them? How to do it?
As Vim is a text editor (not a specialized diff viewer), the full contents of both files are kept (so you can do edits and persist them). Vim usually just "hides" multiple identical lines by folding them away; they are still present, just not visible.
You can influence how many identical lines are kept around changes (default: 6 lines above and below) via the context value of the 'diffopt' option. So, to completely fold all identical lines:
:set diffopt+=context:0