You could use a MemoryStream for example.
MemoryStream stream = new MemoryStream();
using (XmlWriter writer = XmlWriter.Create(stream))
{
// Do some stuff with writer
}
stream.Seek(0, SeekOrigin.Begin); // Reset stream position to read from the beginning.
using (XmlReader reader = XmlReader.Create(stream))
{
// Do some stuff with reader
}