how can i fetch google adsense ads with jquery ajax and display them...
I do not think you can. If you could, I am sure it would be against Google Adsense TOS and be considered modifying the ads.
Stand corrected:
http://stackoverflow.com/questions/1142861/jquery-append-google-adsense-to-div
However, I would be sure to read through the TOS and make sure that is not against it, it is not worth being banned for life from Google Adsense over this, in my opinion.
You don't.
Google has a strict policy as how to display adds. You should use their javascript snippet and their's alone. Everything else will be against the TOS and you'll probably get banned.
You could eventually use the Adsense API (as mentioned) but you should be advised that you are only eligible to do so after you get 3M pageviews/month. This 3M pageviews is just another protection level so that Google knows you'll be playing by the rules.
From the Google Adsense API site:
http://code.google.com/p/google-sensapility/source/browse/trunk/samplecode/v3/GenerateAdCode.php