We are using lr_start_transaction & lr_end_transaction for capturing response time.

Kindly help if this is response time is only server response time for that request or all of the below mentioned?

is it only server response time?

or 

request creation time, request sent time, Connection to server & server time - all these are added to the response time?

Hello Veera
In the transaction we record using the start time (lr_start_transaction), the end time (lr_end_transaction),
In this we measure the end to end times and also any think time that would be configured in between, and wasted time is also recorded.

To get more granular data points and allowing you to see a breakdown of the request, you need to use Web page diagnostics.
https://admhelp.microfocus.com/lr/en/24.1/help/WebHelp/Content/Analysis/109500_toc_Web_Page_Diagnostics_graphs.htm
This will record more layers inside of the request but please note that it will only record for up to 10% of all vusers due to the amount of measurements it will collect .

Thanks David