Hi Rahul,
Below blog explain you in detail how DRS takes migration decision:
DRS Deepdive - Yellow Bricks : It is worth to read twice.
If the earlier VM is migrated to other host and if it is not coming back to original host, it is mean that as per DRS load balancing algorithm CPU/Memory load is balanced and you need not to worry.
However, if you want to keep that VM always on original host, you need to configure DRS affinity rule.
Refer:http://blog.pluralsight.com/vmware-storage-drs-rules
For above requirement, you need to lokk for VM-Host affinity rule.
Let me know if you have any other doubt.