I am an svn user (I am using kdesvn) and I have deleted some files. What can I do If I want to restore them back? I know that the svn server keeps all the files (even if are removed from the svn list)
Just do an update of that file. By default you will get HEAD, but you can specify any previous version. That is if you have not deleted the metadata in the .svn directory. If you have, then you have to checkout the project again.
Get a previous revision of course. Committing an empty file is just a change like any other, just you have changed the whole file. Even rm’ed files can be recovered, they exist in previous revisions of the repo.