Ivanhoe is definitely narrated in the third person, since there is no "I" speaking directly to the reader. The narration is omniscient, or all-knowing: the narrator is aware of everything, from the specific customs and clothing of the Saxons and the Normans to the innermost thoughts and feelings of all the characters.
But even if the narrator sees everything, that doesn't mean he shows it to us. The narrator clearly knows everything that's going on behind the scenes and never expresses any confusion or uncertainty about the novel's events. The narrator also withholds key information to build suspense. The identities of the Black Knight, the Disinherited Knight, and the captain of the outlaws all remain hidden until the other characters in the book discover them. While these characters remain anonymous, we don't get any insight into their thoughts and feelings. The narrative point of view may be omniscient, but the book still builds a lot of drama by choosing when and where it's best to reveal key pieces of information to the reader.