Hi Ameer,
If you want to import the info via flat file I would suggest to read Jothivenkatesh M blog:
http://scn.sap.com/community/crm/master-data-and-middleware/blog/2013/06/27/lsmw-and-xif--idoc-method
or Stephen Johannes blog:
http://scn.sap.com/people/stephen.johannes/blog/2005/08/18/external-data-loads-for-crm-40-using-xif-adapter
For better performance during the data transfer I also would include the LSMW project inside SXDA, SXDA is a datatransfer framework with a set of utilities which will made the datatransfer process easier (pralelizaiton, correction of wrong idocs, reprocess, etc).
Those methods. LSWM with or without SXDA, as far as I know doesn't support XML, so if you plan to upload an XML via this techniques i'm afraid you will need to create a custom program to map de xml to the Bdoc format and call the XIF FM, in Sephen's blog explains how you can search for the XIF interface corresponding to your object.
I now there's another way via smoeac to create a BP ibound subscription with XIF-XML but I never tried, also for huges amounts of data I don't think is the best option (as far as I know doesn't support paralelization in conclusion, the system can become a bottleneck) .
Cheers!
Luis