JQuery ajax() get xml response text

data is not an xhr object, it is your xml string converted into an XML Document. Therefore, it doesn't have a responseText property unless the xml doc has a responseText node. Also, add dataType: "xml" to your ajax options if you are expecting xml.

$.ajax({
    url: "https://mail.google.com/mail/feed/atom/",
    dataType: "xml",
    success: function(data) {
        console.log(data);
    }
});

Edit: Now i see in your question (after edit) that it is infact an xhr object... That's odd...

If you just want a text format response to show, you can simply do dataType: "text",

$.ajax({
  url: "https://miranda-zhang.github.io/cloud-computing-schema/v1.0/ontology.xml",
  dataType: "text",
  success: function(text) {
    $('textarea').val(text);
  },
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<textarea cols="60" rows="10"></textarea>

Try data.responseText[0] instead of data.responseText.

EDIT: https://mail.google.com/mail/feed/atom/ It asks me to log in.