Is it useful to learn CVS before learning SVN? Does much of that knowledge carry over to SVN or are they too different?
Both CVS and SVN are centralized version control systems. Having existing knowledge of workflows and using one will help you use the other, but learning CVS first simply to use SVN wouldn't add any value. If you are interested in a centralized version control system or your team is using SVN, just jump right into SVN. Documentation and tutorials abound on the Internet.
No. Learning CVS is not useful and it will rot your brain and teach you bad habits. Recall that CVS is basically a wrapper for RCS which is about as poor as SCCS. SVN replaces all the functionality of CVS and removes the limitations.
The better bet is to learn SVN and also learn one other system which is decentralized such as mercurial or git.