If i am using action attribute in form tag it'll works fine to interact with servlet, but not with ajax.
Then the problem is not Java related. It's Javascript related and for that you should ask for better help in a Javascript or even better Dojo forum.
I have programmed an Ajax test in Dojo where I call a sevlet and it worked, but I have used a different method "request" which is described in dojotoolkit.org documentation "Ajax with dojo/request". So try it out with your sevlet. If it works, then the problem is not server side related, but client side.
As a general advice, you should insert console.log() message at every line of your Dojo program and see in Firebug if the request is really made. (especially one inside function postform). Firebug has a tab where you can see all requests and its status.
If the request was made at all, show inside your Apache (or IIS or whatever server you have) logs if it really arrives and will be processed or will be rejected as an error.