A difficult one Potsy but I have to say this was rather unprofessional of HR to correspond with your boss in this way given you have access to his emails.

Don't jump to conclusions though.  It may not be a case of a bad performance review but rather a "could do better" one.

Have you had a difficult year?  Personally I would wait until you are called to the review.  I would not instigate a situation were you might cause worry for yourself unnecessarily.

However that said, everyone is different and if it would put your mind at rest have a quiet word with your boss to say that you have seen the email and see what he/she has to say.