var strxml ="<?xml version=\"1.0\"?>"+
                "<PERSONELLER>"+
                "<PERSONEL>"+
                "<ADI>ERCAN</ADI>"+
                "<SOYADI>AKAR</SOYADI>"+
                "</PERSONEL>"+
                "</PERSONELLER>";

            var ms= new MemoryStream(Encoding.UTF8.GetBytes(strxml));
            var xmlDoc = XDocument.Load(ms);
            var nodes= xmlDoc.DescendantNodes();
            var p= nodes.Where(f => f.GetType() == typeof(XElement)).Cast<XElement>()
                .FirstOrDefault(f => ((XElement) f).Name.LocalName.Equals("PERSONEL"));

            textBox1.Text = ((XElement) p.Nodes().ToList()[0]).Name.LocalName; //ad
            textBox2.Text = ((XElement)p.Nodes().ToList()[1]).Name.LocalName; //soyad