I have stack overflow data dump file in .xml format,nearly 27GB and I want to convert them in .csv file. Please somebody tell me, tools to convert xml to csv file or python program
Use one of the python xml modules to parse the .xml file.  Unless you have much more that 27GB ram, you will need to do this incrementally, so limit your choices accordingly.  Use the csv module to write the .csv file.
Your real problem is this. Csv files are lines of fields. They represent a rectangular table.  Xml files, in general, can represent more complex structures: hierarchical databases, and/or multiple tables.  So your real problem to to understand the data dump format well enough to extract records to write to the .csv file.
How to convert xml file of stack overflow dump to csv file