Hello Atul,
You should use the getKeywordVariations() method of the
KeywordToolService:
http://www.google.com/apis/adwords/developer/KeywordToolService.html#getKeywordVariations
Unfortunately, this will provide the variations, but it will not
provide estimated search volume. You can approximate that value by
running the keywords through the TrafficEstimatorService and get the
estimated click volume, though it's obviously not the same data.
Cheers,
-Jeff Posnick, AdWords API Team