If your traceMessage Element have always Error word inside then you can filter based on string comparison if it contains Error then it's Error message.
But I would suggest you to add one attribute to Message Element like to show a category whether it's error or Info.
like
<Message Category="Error">
...
...
</Message>
<Message Category="Info">
...
...
</Message>
It will be easier to apply LINQ to filter data.
[Update]
Ah!! Thanks for updating raitu, I can suggest you following query that might help you.
var resultXML = from c in TraceMessage.Elements("Message")
where ((string)c.Element("traceMessage")).Contains("Error")
select new {
Timestamp = (String)c.Element("timestamp"),
Level = (string)c.Element("Level"),
PID = (string)c.Element("PID"),
traceMessage = (string)c.Element("tracemessage") };