Is it just that one is web based (BI Platform Community Edition) and the other is an application (Kettle). Can Kettle be access through a web browser?
BI Platform/Server is a web platform for performing data mining and running reports. From the Pentaho community website:
"...the Pentaho BI Server which functions as a web based report management system, application integration server and lightweight workflow engine (action sequences.)"
While it can be used to create action sequences, it lacks the features of Kettle, which is a fully-fledged desktop ETL tool. Kettle provides none of the analysis or reporting functionality found in the BI server, and BI server lacks the ETL capabilities. They are two very different tools.