If Kha'Zix, Evolving Hunter attacks a battlefield using his ability by spending 3xp targeting Diana, Lunari - can the Diana player resolve Diana's ability and use the spell that was drawn, before Kha'Zix ability does damage, if it's a reaction?

In your scenario, Diana’s ability resolves before Kha'Zix’s, allowing the Diana player to play a [Reaction] speed spell before the damage is dealt, provided the following sequence is followed:

The Sequence of Events

1. Showdown Begins: When Kha'Zix, Evolving Hunter enters a battlefield where Diana, Lunari is present, a showdown begins.
2. Triggers: Two triggered abilities are placed on the chain simultaneously:
   • Kha'Zix's Trigger: "When I attack, you may spend 3 XP to deal damage..."
   • Diana's Showdown Trigger: "When a showdown begins here, you may pay [1]..."
3. Ordering on the Chain: Per FAQ #2439 and FAQ #2251, triggers are placed on the chain in Turn Player order, then Non-Turn Player order. Assuming the Kha'Zix player is the Active Player:
   • The Kha'Zix player's trigger is placed on the chain first (bottom).
   • The Diana player's trigger is placed on top of it.
4. Reaction Window: Before any trigger resolves, both players may play [Reaction] speed cards (Rule 406.4).
5. Resolution (LIFO): The chain resolves in Last-In, First-Out order:
   • Diana's trigger resolves first: If the Diana player pays [1], they [Predict] and potentially draw a spell.
   • Kha'Zix's trigger resolves next: If the Kha'Zix player chooses to spend 3 XP, damage is dealt.

Ruling

Yes, the Diana player can resolve Diana's ability and use the drawn spell before Kha'Zix's ability deals damage, provided it is a [Reaction] speed spell.

Because Diana's ability is on top of the chain, it resolves before Kha'Zix's. If the drawn card is a [Reaction] spell, it can be played in response to the chain items before they resolve or after Diana's trigger finishes resolving.

Important Constraints

• Chain State: You cannot play [Action] speed spells while the chain is "Closed" (not empty). Per FAQ #9933 and FAQ #6548, [Action] speed spells require an "Open" game state. You are restricted to [Reaction] speed cards until the chain is empty.
• Timing: If you draw a non-Reaction spell, you must wait until the chain is empty and the game state returns to "Open." By that time, Kha'Zix's ability will have already resolved.

Note: I could not fully verify this ruling against the retrieved rules, as the specific text for Kha'Zix, Evolving Hunter was not found in the provided context.