How to process notice to a respondent who is a us citizen and is in us for last 20 years

If you know his USA address, you can mention the same in the summons so that the summons may be sent to his USA address accordingly through court by post or courier service at the cost of the petitioner. 

Court summon can be sent to U.S citizen directly  by registered post under provision of order 5 rule 25 CPC.

Is there a process to get service through the embassy? Because normal post service anyone can avoid.

Indian embassy is not a post office, they will not entertain any such communications until government of India passes any order in this regard.

The court can send the summons directly provided you pay the postal or courier charges for this purpose along with the process memo

The Court shall have to send the summons through Min of Extrnal Affairs diplomatic / consulate bag.

Besides this, It may also be served through email and whatsapp (if court approves). 

Hi Shilpa, I'm Niharika Lohan, an advocate practising in New Delhi. I've read your query and here is my take on it.

Various High Courts have laid down guidelines for issuing documents (notice, summon, etc) to persons residing beyond the territories of India.

In lay man terms, you cannot directly send a notice from your end to such a citizen. You will have to go the Registrar through Registry but not directly.

Additionally, you can file an application seeking Court’s leave (permission) for issuing a notice electronically via e-mail, WhatsApp, etc.

I hope this information helps. You can contact me for further guidance at niharikalohan@yahoo.com