I was wondering if it is ok to present the same paper (may be already published in journals or unpublished) in different conferences? Assuring that the conference will not publish the whole research work but only abstracts in the proceedings.
It depends on the rules of the conference, but it is usually OK. You could get different input from different audiences. Beware of publishing rights if there are published proceedings and you expect the paper to be published in a Journal that expects original and never before published before material.
I think it depends on the field. For example my supervisor (in computer science) tends to give the presentations on our software project using the same theoretical core, but each time this core is supported by different evaluation benchmarks. In my opinion I do not see anything wrong in presenting the same ideas on different conferences, especially if it is not published and one does not try to present it as completely novel work. Diversity of presentation venues facilities better work dissemination after all.