Both are first generation penicillins
Pencillin G => IV or IM
Penicillin V (phenoxymethylpenicillin) => more acidic, more stable and can be given orally (PO). Penicillin VK is just its potassium salt.
NOTE: Penicllin V is less active against gram-negative bacteria then than penicillin G .
Nathwani D, Wood MJ. Penicillins. A current review of their clinical pharmacology and therapeutic use. Drugs 1993;45:866-94.