[SalesForce] converting xml response using Document class and xmlNode

You should check if the element has any value before appending it to your resultFile.

Change this: '\nElement Value:'+xnode.getText();

To: (String.isBlank(xnode.getText()) ? '' :'\nElement Value:'+xnode.getText());

So your controller would now look like this:

public class ParseTest3 {

    public String parsingXmlFile{set;get;}
    public String resultFile{set;get;}

    public void parseFile(){

        Dom.Document doc = new Dom.Document();
        doc.load(parsingXmlFile);
        Dom.XmlNode node = doc.getRootElement();
        resultFile = 'Root Element: '+node.getName();
        if(node.getAttributeCount() > 0) {
            for(integer i=0; i< node.getAttributeCount() ;i++) {
                resultFile += '\nAttribute Name: '+node.getAttributeKeyAt(i)+'\nAttribute Value: '+node.getAttributeValue(node.getAttributeKeyAt(i), null);
            }
        }

        for(Dom.XmlNode xnode : node.getChildElements()) {
            resultFile +='\nElement Name:'+xnode.getName()+(String.isBlank(xnode.getText()) ? '' :'\nElement Value:'+xnode.getText());
            if(xnode.getAttributeCount() > 0) {
                for(integer i=0; i< xnode.getAttributeCount() ;i++) {               
                    resultFile += '\nAttribute Name: '+xnode.getAttributeKeyAt(i)+'\nAttribute Value: '+xnode.getAttributeValue(xnode.getAttributeKeyAt(i), null);
                }
            }
            for(Dom.XmlNode axnode : xnode.getChildElements()) {
                resultFile +='\nElement Name:'+axnode.getName()+(String.isBlank(axnode.getText()) ? '' :'\nElement Value:'+axnode.getText());
                if(String.isNotBlank(axnode.getNamespace()))
                    resultFile += '\nName Space: '+axnode.getNamespace();
                if(axnode.getAttributeCount() > 0) {
                    for(integer i=0; i< axnode.getAttributeCount() ;i++) {               
                        resultFile += '\nAttribute Name: '+axnode.getAttributeKeyAt(i)+'\nAttribute Value: '+axnode.getAttributeValue(axnode.getAttributeKeyAt(i), null);
                    }
                }
            }
        }
    }
}

One more thing. Your current solution will work for XML files that are only 2 levels deep. I'd suggest to change it up a little bit so that it uses a recursive function:

public class ParseTest3 {

    public String parsingXmlFile{set;get;}
    public String resultFile{set;get;}

    public void parseFile(){

        Dom.Document doc = new Dom.Document();
        doc.load(parsingXmlFile);
        Dom.XmlNode node = doc.getRootElement();
        resultFile = 'Root Element: '+node.getName();
        if(node.getAttributeCount() > 0) {
            for(integer i=0; i< node.getAttributeCount() ;i++) {
                resultFile += '\nAttribute Name: '+node.getAttributeKeyAt(i)+'\nAttribute Value: '+node.getAttributeValue(node.getAttributeKeyAt(i), null);
            }
        }

        resultFile += parseRecursive(node.getChildElements());
    }

    private String parseRecursive(List<Dom.XmlNode> nodes){
        String retVal = '';

        for(Dom.XmlNode n : nodes) {
            retVal +='\nElement Name:'+n.getName()+(String.isBlank(n.getText()) ? '' : '\nElement Value:'+n.getText());
            if(String.isNotBlank(n.getNamespace()))
                retVal += '\nName Space: '+n.getNamespace();
            if(n.getAttributeCount() > 0) {
                for(integer i=0; i< n.getAttributeCount() ;i++) {               
                    retVal += '\nAttribute Name: '+n.getAttributeKeyAt(i)+'\nAttribute Value: '+n.getAttributeValue(n.getAttributeKeyAt(i), null);
                }
            }
            retVal += parseRecursive(n.getChildElements());
        }

        return retVal;
    }
}

Now your controller will be able to parse any XML string you pass to it, and you won't have to add additional nested for loops to parse additional levels of XML nodes.

You can set as many namespaces as you'd like with setNamespace. Here's a trivial execute anonymous script:

Dom.Document doc = new Dom.Document();
Dom.XmlNode root = doc.createRootElement('root','http://defaultns/','');
root.setNamespace('a','http://example-ns1/');
root.setNamespace('b','http://example-ns2/');
System.debug(doc.toXmlString());

Output (note: This is cleaned up for legibility):

<?xml version="1.0" encoding="UTF-8"?>
  <root xmlns="http://defaultns/" 
        xmlns:a="http://example-ns1/" 
        xmlns:b="http://example-ns2/" />

Observe how root now has three namespaces, the default namespace, plus "a" and "b" namespaces.