Document.createComment()

CommentNode = document.createComment(data);

var docu = new DOMParser().parseFromString('<xml></xml>',  'application/xml');
var comment = docu.createComment('This is a not-so-secret comment in your document');

docu.getElementsByTagName('xml')[0].appendChild(comment);

alert(new XMLSerializer().serializeToString(docu));
// Displays: <xml><!--This is a not-so-secret comment in your document--></xml>