We report measurements of proper motion, radial velocity, and elemental composition for 14 X-ray knots in Keplers supernova remnant (SNR) using Chandra data. The highest speed knots show both large proper motions (0.11-0.14 /yr) and high radial velocities (v ~ 8,700--10,020 km/s) with estimated space velocities comparable to the typical Si velocity (~10,000 km/s) seen in SN Ia near maximum light. High speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Keplers SNR to a consistent central position that agrees well with previous determinations, but is less subject to systematic errors. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 <~ m <~ 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through relatively low density (n_H < 0.1 cm^-3) regions in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for SN Ia models. Other knots show slower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SN Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.
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