guid must be a full URL, unless isPermaLink attribute is false
By default, the guid element specifies a permanent link for an
item, the value must be a full URL (for example, starting with
"http://").
URIs (such as tag: URIs), as opposed to URLs, only identify,
not locate, items, and are not suitable for permalinks.
If you are using the guid element as a permanent link, make sure
the value is a full URL, that could be followed directly in a browser.
If you are using the guid element simply as a unique identifier
(and not a link to the item), then the value can be whatever you want, but you
must include the attribute isPermaLink="false" in the
guid tag.
These are valid guids: