The datomic_kvs table was introduced with the udm_manager probe in UIM 8.1 and holds the information about the udm data-store (device and topology information / history) .
Datomic peers use this data-store to retrieve information and include the discovery_server, UGS and USM. Since 8.4 the functionality of the UGS has been taken over by the trellis probe, and the use of the datomic_kvs table has been optimized.
So it is expected that the table will grow, but abnormally large growth of this table can indeed cause issues. A large table could also be an artifact from a previous version of UIM where the management of this table was not yet optimized.