A Zero reducer as the name suggests omits the reducing step and the final output is only the map part of MapReduce. It is also known as a Map-only step.
As far as Identity Reducer is concerned, the input Key, Value pairs are just dumped into the output as it is without any aggregation, except the data is sorted based on the key. Here, shuffle & sort happens except the aggregation. Hence, Identity Reducer is used if you only want to sort the input data from map output.