How can i maintain the curvature of the main object while adding details with subdivision modeling without destroying the curve?
Depends on how you are adding the details. But one of the nifty tricks is to use the Shrinkwarp modifier underneath with the duplicate of the original
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Of course 
, totally forgot about this trick with the Shrinkwarp modifier, Thanks! 
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Another trick to avoid the non smooth shading is to make splits and or support loops on coplanar faces rather than at pointed edges. For example, the raised block extremities would lie on the middle of faces rather than their boundaries / edges.
In addition to @kkar’s useful solution, I also make use of these two add-ons a lot when I need to smooth edges, edge loops and surface areas:
EdgeFlow:
Volume Preserving Smoothing:
Couldn’t live without those two anymore for organic polygon modeling.