What is the best backup method for CVS full daily or daily differential backup then weekly full?
Is there any problems of using differential backup for cvs repositories ?
A CVS repository is just a collection of files, so the best backup is one you know will restore reliably, there are no special requirements.
Most CVS repositories will not be large enough for a differential or incremental backup to be worthwhile so on pure management grounds a full backup is best.
The only likely problem with a differential file backup is that some of them don't delete files that are in the full backup but don't exist at the time of the differential. This can create (minor) corruption of the repository, for example a file both in the the main database and the 'Attic'.
CVS repositories are good candidates for data transfer using rsync(1) but a continuous backup scheme where you have access to a lot of previous versions of the files is probably useless as CVS already does that.
Lastly, and a little off-topic, CVS is now basically an obsolete technology, you should change to a more recent VCS as they all give you much better visibility of the history of the project and give you much better tools for working with multiple users. My recommendation for both single and multiple user would be usually be git.