I want to save the a BPMN model into an executable XML format. What to choose between XPDL,BPEL,BPMN 2.0 XML. The XML file is going to be executed by a workflow engine.
The executable format you need will depend on what the workflow engine you use understands. Having said that: the only format in which you really can "save" the BPMN model without loosing or modifying some of your BPMN execution semantics is BPMN XML, because it's directly defined in the BPMN spec and therefore you don't have any mapping issues in between formats. Nowadays, the process engines of the bigger commercial vendors (IBM, Oracle, SAP) support BPMN XML, and the Open Source process engines in the Java space (Activiti, Bonita BPM, Camunda BPM, jBPM) support it, too.