Both on python2.7 and python3.4
>>> from xml.etree import cElementTree as ET
>>> text = '<end>its &gt; &lt; &amp; &apos;</end>'
>>> root = ET.fromstring(text.encode('utf-8'))
>>> ET.tostring(root, method="xml")
<end>its &gt; &lt; &amp; '</end>

I would expected to return the same as the input to be a complient XML 1.0

I would understand why for html it would return something diffrent, see:
http://stackoverflow.com/questions/2083754/why-shouldnt-apos-be-used-to-escape-single-quotes

as a workaround I had to path ElementTree:

from xml.etree.ElementTree import _escape_cdata ,_raise_serialization_error
from mock import patch

def _escape_cdata(text):
    # escape character data
    try:
        # it's worth avoiding do-nothing calls for strings that are
        # shorter than 500 character, or so.  assume that's, by far,
        # the most common case in most applications.
        if "&" in text:
            text = text.replace("&", "&amp;")
        if "<" in text:
            text = text.replace("<", "&lt;")
        if ">" in text:
            text = text.replace(">", "&gt;")
        if "'" in text:
            text = text.replace("'", "&apos;")
        return text
    except (TypeError, AttributeError):
        _raise_serialization_error(text)

from xml.etree import cElementTree as ET

text = '<end>its &gt; &lt; &amp; &apos;</end>'
root = ET.fromstring(text.encode('utf-8'))

with patch('xml.etree.ElementTree._escape_cdata', new=_escape_cdata):

    s = ET.tostring(root, encoding='unicode', method="xml")
print(s)