It is not a good thing to have to specify DIP or SP again by code when already defined in a dimen.xml file.
I think that the best option is to use PX when using a dimen.xml value :
tv.setTextSize(TypedValue.COMPLEX_UNIT_PX, getResources().getDimensionPixelSize(R.dimen.txt_size));
This way, you can switch from DP to SP if needed in dimen.xml file without having to change any code.