If two revision numbers, separated by a colon, are passed via --revision (-r), then the two revisions are directly compared.

$ svn diff -r 2:3 rules.txt
Index: rules.txt
===================================================================
--- rules.txt	(revision 2)
+++ rules.txt	(revision 3)
@@ -1,4 +1,4 @@
 Be kind to others
-Freedom = Chocolate Ice Cream
+Freedom = Responsibility
 Everything in moderation
 Chew with your mouth open
$

A more convenient way of comparing a revision to the previous revision is to use the --change (-c) option:

$ svn diff -c 3 rules.txt
Index: rules.txt
===================================================================
--- rules.txt	(revision 2)
+++ rules.txt	(revision 3)
@@ -1,4 +1,4 @@
 Be kind to others
-Freedom = Chocolate Ice Cream
+Freedom = Responsibility
 Everything in moderation
 Chew with your mouth open
$

Lastly, you can compare repository revisions even when you don't have a working copy on your local machine, just by including the appropriate URL on the command line:

$ svn diff -c 5 http://svn.example.com/repos/example/trunk/text/rules.txt
…
$