Question

What is the difference between serving a page as text/xml and application/xhtml+xml ?

Answer 1

Quote from the specification:

The ‘application/xhtml+xml’ media type [RFC3236] is the primary media
type for XHTML Family documents. ‘application/xhtml+xml’ should be
used for serving XHTML documents to XHTML user agents (agents that
explicitly indicate they support this media type). This media type
must be used when writing documents using XHTML Family document types
that add elements and attributes from foreign namespaces, such as
XHTML+MathML [XHTML+MathML].

So use application/xhtml+xml to serve your XHTML pages if the client supports it, otherwise you could use text/html.

Another quote from the specification:

1. If the Accept header explicitly contains application/xhtml+xml (with
   either no “q” parameter or a positive “q” value) deliver the document
   using that media type.

2. If the Accept header explicitly contains
   text/html (with either no “q” parameter or a positive “q” value)
   deliver the document using that media type.

3. If the accept header
   contains “* /*” (a convention some user agents use to indicate that
   they will accept anything), deliver the document using text/html.

In other words, requestors that advertise they support XHTML family
documents will receive the document in the XHTML media type, and all
other requestors that (at least claim to) support HTML or “everything”
will receive the document using the HTML media type. Dealing with user
agents that satisfy none of these criteria is outside the scope of
this document.

When an XHTML document does NOT adhere to the guidelines, it should
only be delivered as media type application/xhtml+xml.

text/xml should not be used for serving XHTML web pages. You could use it to serve other XML documents that do not represent XHTML.