In R2RS, exact numbers are eqv? if they are =.  All other cases are
implementation-defined.

In R3RS, a notion of operational equivalence is defined, and numbers
are operationally equivalent (and therefore eqv?) iff they are = and
have the same exactness.

In R4RS, the notion of operational equivalence is dropped, and eqv? is
directly defined on numbers in the same way as in R3RS.

R5RS continues R4RS.

In R6RS, operational equivalence is restored as the criterion, though
the name is not used.  Rational numbers are eqv? iff they are = and
have the same exactness.  Otherwise, they are eqv? if they are =;
they are not eqv? if they are operationally distinguishable or have
different exactness.  They are indeterminate otherwise.

In R7RS draft 6, numbers are eqv? iff they are = and have the
same exactness, except that if one of them is +nan.0 the result is
indeterminate.

--
"The serene chaos that is Courage, and the phenomenon   cowan@ccil.org
of Unopened Consciousness have been known to the        John Cowan
Great World eons longer than Extaboulism."
"Why is that?" the woman inquired.
"Because I just made that word up", the Master said wisely.
        --Kehlog Albran, The Profit             http://www.ccil.org/~cowan