Getting all xml files in a folder and subfolders

Question

Currently I'm able to get and return files from a directory. However, it cannot get only xml files and get files from sub-folders.

Example folder:

folder
   |_ folder1
      |_file1.xml
      |_file2.xml
   |_ folder2
      |_file3.xml
 file4.xml

Expected return:

file1.xml
file2.xml
file3.xml
file4.xml

My current code is like this

def get_files(d):
    return [os.path.join(d, f) for f in os.listdir(d) if os.path.isfile(os.path.join(d, f))]

get_files(*filepath*)

And the code only returns file4.xml