Lv.

It totally means Level. What are you talking about?

...Level of violence, not experience. Geez, we gotta keep the "genocide vs pacifism" shtick going here, don't we?

Lv. is a measure of how violent you are and/or can be. It is not a judge of one's character, unless you're Sans, in which case, an Lv. higher than 19 means you should burn in hell. Some see Lv. as a score, others as a haunting reminder of their Sins. Yes, there is an entire page for sins. Sin's a recurring theme in Undertale.