Slowly but surely, India is piecing together what seems to be a 2-tier missile defense network, but a 3-tier system may not be far away.
The PDV (which I will assume will eventually replace the PAD) seems to be a very close comparator to the THAAD, able to intercept satellites and warheads up to 150 km in altitude (I suspect the actual apogee is higher, but the official figure will do for now). The implications being, of course, that India would have a shield against MRBMs and IRBMs for terminal interception, but most importantly it would be able to intercept SRBMs and TBMs at their midcourse stage, allowing for very high probability of intercept (keep in mind that SRBMs and TBMs rarely have decoys). Satellites, especially those in very low LEO, would fall prey to the PDV as well. For now, until the advent of high-altitude ICBM-specific interceptors (which I will outline below), the PDV will form the first layer of defense for India's BMD network.
The AAD (Ashwin) seems to be an endoatmospheric interceptor, straddling between the S-400 and PDV in terms of ABM capability. I expect that it would eventually complement the S-400 to deal with SRBMs, TBMs, and occasionally IRBMs (i.e. anything that manages to bypass the PDV). This is more or less comparable to the PAC-3 ERINT/MSE but without a KKV.
India has not overtly embarked on a project to develop very-high-altitude midcourse interceptors (i.e. ones similar to the US GBI or the Chinese SC-19 & DN-1/2/3), but I believe existing technologies brought about by its space program would allow it to do so. To intercept ICBMs at their midcourse stage, an ABM must reach altitudes of 1000-2000 km (before the RV separates from their bus). Two rocket boosters are candidates for such a missile: Satellite Launch Vehicle (SLV) and Augmented Satellite Launch Vehicle (ASLV). Their respective masses (17 & 41 tons, respectively) are roughly comparable to the US GBI and the Chinese SC/DN series. Their launch history also demonstrates their ability to place 40-50 kg satellites (roughly the same weight as a kinetic kill vehicle) into high altitudes (800-1000 km). The final piece of the puzzle is the seeker to distinguish the RV & missile from other objects in space; this should not be a problem since the PDV has already demonstrated this.
On a final note, India needs to develop a high-altitude midcourse interceptor similar to the US GBI if it wants a decent chance of intercepting MIRVed ICBMs & IRBMs. On a positive note, almost all of its major subcomponents are in place for DRDO to develop them (granted, with Israeli radars, of course).