What happens to the I/O pins during power-up?
The IOBs will remain 3-stated through power-on if the recommended power-on sequence is followed. The recommended power-on sequence is VCCINT, VCCAUX, and VCCO, or VCCINT, followed by VCCIO for earlier devices that did not have a VCCAUX rail. They will be held in this 3-state until the power-on reset pulse has completed.
In the Virtex family (i.e., XCVxxx), the mode pins are sampled as soon as the power is applied. The mode pins determine whether the IOB pull-ups will be added or if they are left floating prior to configuration. From the Virtex-II devices onwards, the HSWAP_EN pin was introduced that controlled if the IOB pull-ups are enabled preconfiguration. If HSWAP_EN is not applied, the IOBs are left floating.