My manager has given a diplomatic feedback which I'm not happy with,what should I say in response?
Explanation of Feedback from Manager
Since this is based on a performance review, did you ask for an explanation on the feedback your manager gave on your review? If not, this is where I'd start. It's quite possible the feedback is justified or has been miscommunicated to you - Understanding exactly why your manager feels the way he or she does is the first step in this process.
Why are you not happy?
Once you've received an explanation of the feedback, ask yourself why you're not happy about it? Is the feedback justifiable? Are you not happy based on it purely being negative feedback, or is it that you're not happy based on the feedback itself in which you feel isn't true?
If it's purely based on it being negative feedback, then perhaps it's not negative feedback at all, but constructive criticism? If you feel the feedback doesn't represent what you've done since your last review, then...
What can you do to rectify the feedback?
A great, professional way of taking such feedback and putting it to good use is to ask your manager to help you create a plan of action on how this feedback can be turned into positive feedback. What steps can be taken to make sure this never shows back up on your performance review again? Is it possible to get the previous review amended once all of these steps are completed?
Ultimately, talk to your manager - but go about it in a professional manner, try putting yourself in their shoes to see their side of the story - If it's still unjustified then you must ask yourself is it enough of a blemish to take actions a step further, or is it a small enough disagreement that it can be looked over. If the feedback is really getting to you and you think it's important then explain this to your boss, if he/she refuses such - then take it to his superior or an HR representative. (Noting that this would need to be a pretty extreme case for me to do the last advice given, and only in the most dire of circumstances - such as career-ending.)