lxml, get xml between elements

given this sample xml:

<xml>
<pb facs="id1" />
  <test></test>
  <test></test>
<pb facs="id2" />
  <test></test>
  <test></test>
</xml>

i need to parse it and get the content between pb

what is the correct xpath axe to use?

from lxml import etree
xml = etree.parse("sample.xml")

for pb in xml.xpath('//pb'):
    print pb.xpath('@facs')[0]
    print {{ HOW TO GET THE CONTENT HERE? }}