void Main()
{
   String aciResponseData = "<?xml version=\"1.0\" encoding=\"UTF-8\"?><tag><bar>test</bar></tag>";
   using(TextReader sr = new StringReader(aciResponseData))
   {
        var serializer = new System.Xml.Serialization.XmlSerializer(typeof(MyClass));
        MyClass response =  (MyClass)serializer.Deserialize(sr);
        Console.WriteLine(response.bar);
   }
}

[System.Xml.Serialization.XmlRoot("tag")]
public class MyClass
{
   public String bar;
}

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load("your file path");

C# object to xml document