Вы можете использовать Transformer:
private String nodeToString(Node node) {
StringWriter sw = new StringWriter();
try {
Transformer t = TransformerFactory.newInstance().newTransformer();
t.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
t.transform(new DOMSource(node), new StreamResult(sw));
} catch (TransformerException e) {
e.printStackTrace();
}
return sw.toString();
}