I-134, K-Visas are Non-Immigrant visas, and use I-134.
I-864 is used for situations that result in a green-card being issued, IR-1/CR-1 immigrant visas use I-864, as well as Adjusting status in the USA from the K-Visas. (I-485).
QUOTE
9 FAM 41.81 N4 DOCUMENTARY REQUIREMENTS
b. K-1 and K-3 applicants are subject to INA 212(a)(4) and must demonstrate to the consular officer's satisfaction that they will not become a public charge. The Form I-864, Affidavit of Support Under Section 213 A of the Act cannot be required. Applicants may submit a letter from the petitioner's employer or evidence that they will be self-supporting. The Form I-134, Affidavit of Support, may be required when the consular officer deems it useful.
http://foia.state.gov/searchRegs/vv_docVie...eryText=I%2D864