The SVN plugin used to work reasonably well with CVS back in the old days, although CVS support was always a secondary priority. In fact, the additional code to support CVS was what effectively made the project nearly unmaintainable.
I haven't been working on that plugin for about 2-3 months now, been busy with Code::Blocks core during that time. The next (long awaited) release of the SVN plugin will only support Subversion, and everything related to CVS will be made into a separate, independent module after that.
CVS will always be secondary to me, as more and more projects are moving to Subversion which is a lot better anyway. If you can't wait and need CVS support now, feel welcome to make a plugin of your own (but please let me know, so we don't do the same work twice).