how to insert data into extra columns of target avro table when source table is having less no of columns compared to target using hive or impala?
You need to specify which columns you want to insert into in the insert statement. In the statement you wrote, the engine doesn't know which 7 columns out of the 8 to put the data in.
http://www.cloudera.com/documentation/enterprise/5-5-x/topics/impala_insert.html

Insert overwrite table target_table (col1, col2, col3, ...) 
Select col1, col2, col3 ... From source_table;


NULL will be inserted into columns not mentioned in the column list.