If you are looking for a way to parse xml text in JavaScript, then look here:
XML parsing of a variable string in JavaScript
Info about loading xml file from JavaScript here:
Load xml from javascript
Creating markers from custom xml is then quite simple. I added a short example. It shows how to parse xml text or load xml file and then create markers from the data.
Some custom xml text:
var myXmlText = '<?xml version="1.0" encoding="ISO-8859-1"?>' +
      '<coords>' +
         '<item><position lat="50.1" lng="14.5"/><title>Praha</title></item>' +
         '<item><position lat="51.5" lng="0"/><title>London</title></item>' +
         '<item><position lat="48.8" lng="2.4"/><title>Paris</title></item>' +
         '<item><position lat="52.5" lng="13.4"/><title>Berlin</title></item>' +
         '<item><position lat="48.2" lng="16.4"/><title>Wien</title></item>' +
      '</coords>';
Parsing the xml text into xml DOM object:
function parseXml(xmlText) {
   var xmlDoc;
   if (window.DOMParser) {
      parser=new DOMParser();
      xmlDoc=parser.parseFromString(xmlText,"text/xml");
   } else { // Internet Explorer
      xmlDoc=new ActiveXObject("Microsoft.XMLDOM");
      xmlDoc.async="false";
      xmlDoc.loadXML(xmlText); 
   }
   return xmlDoc;
}
Loading and parsing a xml file into xml DOM object:
function loadXml(xmlUrl) {
   if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp=new XMLHttpRequest();
   } else {// code for IE6, IE5
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
   xmlhttp.open("GET",xmlUrl,false);
   xmlhttp.send();
   xmlDoc=xmlhttp.responseXML;
   return xmlDoc;
}
Create the markers from data in xml DOM object:
function createMarkers(xmlDoc) {
   var items = xmlDoc.getElementsByTagName('item');
   for(var i=0; i<items.length; i++) {
      var positionEl = items[i].getElementsByTagName('position')[0];
      var latlng = new google.maps.LatLng(positionEl.getAttribute('lat'), positionEl.getAttribute('lng'));
      var titleNode = items[i].getElementsByTagName('title')[0].childNodes[0];
      var marker = new google.maps.Marker({
          position: latlng,
          map: map,
          title: titleNode.nodeValue
      });
    }
}
To create the markers from xml file, all you need to do is:
var xmlDoc = loadXml("my_items.xml");
createMarkers(xmlDoc);
To create the markers from the xml text, use:
var xmlDoc = parseXml(myXmlText);
createMarkers(xmlDoc);