For anyone interested in getting fast, cheap affection with Khem Val, simply re-run Black Talon in your down time. You will have to take the Dark Side options to achieve the optimum results, but for most that is not an issue. Simply follow this numbered conversation guide to earn a maximum of 455 affection with Khem Val!

Responses numbered 1-3 represent numerical response. # represents a choice that has no impact on Khem Val.

Conversation 1, Responses: 3, 3, #
Conversation 2, Responses: 3, 2, #, 1
Conversation 3, Response: 1
Conversation 4, Responses: 2, 3, 1, 2, 3, 3
Conversation 5, Responses: 3, #, 1, 3, 1, 2, 3
Conversation 6, Responses: 3, 1
Conversation 7, Response: 3
Conversation 8, Responses: 3, 2, 2, 2
Conversation 9, Response: #
Conversation 10, Responses: 3, 2, #, 2, 2
Conversation 11, Response: #

Aside from the 455 Affection Gain with Khem Val, you will also earn yourself 200 Dark Side Points.

Keep in mind, any responses made by other players could alter the progression of conversations and make this guide inaccurate.

Companion gifts that Khem Val likes:

1 Cultural Artifact (rank 1 gift, 96 points)
2 Weapon (rank 1 gift, 54 points)
3 Trophy (rank 1 gift, 24 points)
4 Imperial Memorabilia (rank 1 gift, 24 points)
5 Technology (rank 1 gift, 24 points)

Khem Val will also give affection points for selecting dialogue options that convey your strength, fearlessness, and elude to killing.

Khem Val is the tanking companion that the Sith Inquisitor obtains in the early levels on Korriban. He generally reacts positively to many dark side choices..

Historically Khem Val was a servant of the Sith Lord Tulak Hord. He enjoys devouring force users and the in-game conversations reflect this. Khem Val is a member of the Dashade species that come from the planet Urkupp.

The following is a small modification on Watchman's post that made my first few initial farms for Khem Val rep in Black Talon easier. I had a problem remembering which conversation was which, and I clarified some conversation points. It's also relatively spoiler free, for those who see this post that haven't done the flashpoint yet.

Responses numbered 1-3 represent numerical response. # represents a choice that has no impact on Khem Val.

lady: 3, 3, #
droid: 3, 2(only non negative response), #, 1
man: 1
droid: 2, 3, 1(dark side), 2, 3, 3
droid: 3, #, 1, 3, 1, 2, 3
computer: 3, 1
door: 3
engine room: 3, 2, 2, 2(dark side)
computer: #
droid: 3, 1/2(3 is negative), #, 2, 2
computer: #

Aside from the 455 Affection Gain with Khem Val, you will also earn yourself 200 Dark Side Points.

By level 28 I have come to the following conclusions about Khem Val.

1) He will approve of almost all Dark side decisions (so far, by the end of Nar Shaddaa, I have found one Dark side decision that he did not approve of, and I lost all of 1 affection. Worth it, I should think.)
2) If you plan on going Dark side all the way, and being arrogant, boastful and murderous about it, you will have no trouble whatsoever pleasing Khem. (Killing Lord Paladius wasn't even a Dark Side decision, and I gained 120 affection! He usually gives 40 for Dark side decisions, and 15 for normal conversation options.)
3) He will disapprove of any and all courtesy you show. (This is what I originally wanted; to strive for power through subtlety, but Khem quickly demolished those plans.)
4) Black Talon really is a brilliant place to grind Khem's affection. (See the comment by Watchman for details.)

Responds favorably to the following types of gifts:
Technology
Imperial Memorabilia
Trophy
Cultural Artifact ***
Weapon ***

*** Giving significantly more than the other types of gifts.

SPOILER ALERT!

If you allow the scientist on Balmorra to go back to the Republic once he's made your serum, you lose a whopping 160 AFFECTION with Khem Val!

I updated the previous versions with a lightside version
lady: 3, 3, #
droid: 3, 2(only non negative response), #, 1
man: 1
droid: 2, 3, 2(light side -30), 3, 3, 3
droid: 3, #, 1, 3, 1, 2, 3
computer: 3, 1
door: 3
engine room: 3, 2, 2, 1(light side -30)
computer: 3
droid: #, #, 2, 2
computer: #

+200 LP
+300 Khem Val rep

Khem Val likes:
--Displaying strength
--"Eating" (a.k.a. killing everything in sight)

Khem Val dislikes:
--Showing any form of weakness
--Allowing people to live
--Force users

I've had good luck with presenting Khem Val with trophy companion gifts.

Gift gain values

+96
Cultural Artifact

+54
Weapon

+24
Trophy
Technology
Imperial Memorabilia

+0
Underworld Good
Courting
Luxury
Republic Memorabilia
Military Gear

This is probably obvious but rank 1 companion gifts (200 credits) give half the value in affection as rank 2 gifts, yet cost a third of the price of rank 2 gifts (600 credits). It is much more efficient for your money to buy rank 1 gifts if you want affection, unless they end-up giving you absolutely nothing (maybe at a very high affection level, I haven't tested).

Khem Val is, unsurprisingly, not very talkative and even at 4500+ affection, does not give any quest or much dialogues. He may react to some story events and that may be his way to interact with you. I was in a full party when doing the Dark Temple story mission and couldn't summon him. I now wonder how he would have reacted to the mention of his former master...