Low SvO2 means the entire body is extracting more oxygen than normal from the blood. High means your body is taking less oxygen from the blood because the vessels are so clamped down they cannot get oxygen into the tissues.
Lactic acid is a byproduct of anaerobic metabolism, when the tissues have used up all the oxygen it can during aerobic metabolism. Everyone has what they call a normal SvO2 but for me it's 60-75%. That means 25-40% of the oxygen pumped out by your heart is being used by the body, 60-75% of the circulated oxygen makes it way back without being utilized. If your SvO2 is, for example, 55, you might not see anaerobic metabolism. Your body is just extracting more oxygen for normal function. At 30 or 40, you're likely going to see anaerobic metabolism and a concurrent rise in lactate or even lactic acidosis.
You can also see a rise in lactate without a drop in SvO2 for many other reasons (i.e. tissue death, respiratory distress, etc.), so you have to know the whole picture to utilize either the SvO2 or lactate properly to treat your patient.